Decrease of P-Glycoprotein Activity in K562/ADR Cells by M?CD and Filipin and Lack of Effect Induced by Cholesterol Oxidase Indicate That This Transporter Is Not Located in Rafts

Cholesterol-Depletion-Induced Membrane Repair Carries a Raft Conformer of P-Glycoprotein to the Cell Surface, Indicating Enhanced Cholesterol Trafficking in MDR Cells, Which Makes Them Resistant to Cholesterol Modifications

The human P-glycoprotein (P-gp), a transporter responsible for multidrug resistance, is present in the plasma membrane's raft and non-raft domains. One specific conformation of P-gp that binds to the monoclonal antibody UIC2 is primarily associated with raft domains and displays heightened internalization in cells overexpressing P-gp, such as in NIH-3T3 MDR1 cells. Our primary objective was to investigate whether the trafficking of this particular P-gp conformer is dependent on cholesterol levels. Surprisingly, depleting cholesterol using cyclodextrin resulted in an unexpected increase in the proportion of raft-associated P-gp within the cell membrane, as determined by UIC2-reactive P-gp. This increase appears to be a compensatory response to cholesterol loss from the plasma membrane, whereby cholesterol-rich raft micro-domains are delivered to the cell surface through an augmented exocytosis process. Furthermore, this exocytotic event is found to be part of a complex trafficking mechanism involving lysosomal exocytosis, which contributes to membrane repair after cholesterol reduction induced by cyclodextrin treatment. Notably, cells overexpressing P-gp demonstrated higher total cellular cholesterol levels, an increased abundance of stable lysosomes, and more effective membrane repair following cholesterol modifications. These modifications encompassed exocytotic events that involved the transport of P-gp-carrying rafts. Importantly, the enhanced membrane repair capability resulted in a durable phenotype for MDR1 expressing cells, as evidenced by significantly improved viabilities of multidrug-resistant Pgp-overexpressing immortal NIH-3T3 MDR1 and MDCK-MDR1 cells compared to their parents when subjected to cholesterol alterations.

The Effects of Iodinated Radiographic Contrast Media on Multidrug-resistant K562/Dox Cells: Mitochondria Impairment and P-glycoprotein Inhibition

Iodinated radiographic contrast media is used in cancer radiography for cancer diagnosis. The aim of this present study was to examine five iodinated radiographic contrast media (IRCM) (i.e., iohexol, iopamidol, iobitridol, ioxaglate, and iodixanol) in terms of their cytotoxicity, mitochondria membrane potential (ΔΨm), and P-glycoprotein function in multidrug resistant K562/Dox cancer cells and corresponding sensitive cancer cells. The cytotoxicity was determined by colorimetric resazurin reduction assay. The ΔΨm and P-glycoprotein function was measured using a noninvasive functional spectrofluorometry. Rhodamine B, fluorescence probe, was used to estimate ΔΨm. The kinetic of P-glycoprotein-mediated efflux pirarubicin was used to monitor P-glycoprotein function in multidrug resistant (MDR) cancer cells. The results showed that ioxaglate and iodixanol show similar efficacy in MDR cancer cells and for their corresponding sensitive cancer cells. Iopamidol, iohexol, and iobitridol showed higher efficacy in MDR cancer cells than for the corresponding sensitive cancer cells by approximately 2 fold. The results also showed no significant change in the |ΔΨm| values in treated K562 and K562/Dox cancer cells when compared to the non-treated K562 and K562/Dox cancer cells. However, there were notable changes detected for iobitridol and iodixanol at 50 mgI/mL. Similarly, the results showed significant differences in P-glycoprotein function of K562/Dox cancer cells after treatment with IRCM when compared to the non-treated K562/Dox cancer cells, with iohexol and iodixanol being the notable exceptions once again. In this present study, IRCM exhibited cytotoxicity on MDR cancer cells and their corresponding sensitive cancer cells. IRCM also showed potential as an anticancer agent in the future.

Fullerene inhibits benzo(a)pyrene Efflux from Cyprinus carpio hepatocytes by affecting cell membrane fluidity and P-glycoprotein expression

P-Glycoprotein (P-gp) can protect cells by pumping out toxic compounds, and has been found widely expressed in fish tissues. Here, we illustrate the P-gp efflux ability for benzo(a)pyrene (BaP) in the hepatocytes of common carp (Cyprinus carpio) after exposing to fullerene aqueous suspension (nC60). The results revealed that nC60 increased the membrane fluidity by decreasing the ratio of saturated to unsaturated fatty acids, and increased the cholesterol contents. These findings, combined with 10-38% and 70-75% down-regulation of P-gp mRNA and protein respectively, suggested that nC60 caused inhibition on P-gp efflux transport system. Therefore, we further investigated the cellular efflux ability for BaP. Results showed unequivocally that nC60 is a potent P-gp inhibitor. The retaining BaP amounts after efflux were elevated by 1.7-2.8 fold during the 10 day exposure. Meanwhile, 5mg/L humic acid (one of the important fractions of natural organic matter, which is ubiquitous in aquatic environment) alleviated the nC60 damage to hepatocytes in terms of oxidative damage, cholesterol increment, and P-gp content reduction; and finally attenuated the suppressed P-gp efflux ability. Collectively, this study provides the first evidence of nC60 toxicity to P-gp functionality in fish and illustrates the possible mechanism of the suppressed P-gp efflux ability for BaP.

Multidrug resistance (MDR) reversers: High activity and efficacy in a series of asymmetrical N,N-bis(alkanol)amine aryl esters

As a continuation of our research on potent and efficacious P-gp-dependent multidrug resistance (MDR) reversers, several new N,N-bis(alkanol)amine aryl esters were designed and synthesized, varying the aromatic moieties or the length of the methylenic chain. The new compounds were tested on doxorubicin-resistant erythroleukemia K562 cells (K562/DOX) in the pirarubicin uptake assay, where most of the new compounds were shown to be active. In particular the asymmetrical compounds, characterized by two linkers of different length, generally showed fairly high activities as MDR reversers. Some selected compounds (isomers 15-17) were further studied by evaluating their doxorubicin cytotoxicity enhancement (reversal fold, RF) on the K562/DOX cell line. The results of both pharmacological assays indicate that compounds 16 (GDE6) and 17 (GDE19) could be interesting leads for the development of new P-gp dependent MDR modulators.

Complex Interplay between the P-Glycoprotein Multidrug Efflux Pump and the Membrane: Its Role in Modulating Protein Function

Multidrug resistance in cancer is linked to expression of the P-glycoprotein multidrug transporter (Pgp, ABCB1), which exports many structurally diverse compounds from cells. Substrates first partition into the bilayer and then interact with a large flexible binding pocket within the transporter's transmembrane regions. Pgp has been described as a hydrophobic vacuum cleaner or an outwardly directed drug/lipid flippase. Recent X-ray crystal structures have shed some light on the nature of the drug-binding pocket and suggested routes by which substrates can enter it from the membrane. Detergents have profound effects on Pgp function, and several appear to be substrates. Biochemical and biophysical studies in vitro, some using purified reconstituted protein, have explored the effects of the membrane environment. They have demonstrated that Pgp is involved in a complex relationship with its lipid environment, which modulates the behavior of its substrates, as well as various functions of the protein, including ATP hydrolysis, drug binding, and drug transport. Membrane lipid composition and fluidity, phospholipid headgroup and acyl chain length all influence Pgp function. Recent studies focusing on thermodynamics and kinetics have revealed some important principles governing Pgp-lipid and substrate-lipid interactions, and how these affect drug-binding and transport. In some cells, Pgp is associated with cholesterol-rich microdomains, which may modulate its functions. The relationship between Pgp and cholesterol remains an open question; however, it clearly affects several aspects of its function in addition to substrate-membrane partitioning. The action of Pgp modulators appears to depend on their membrane permeability, and membrane fluidizers and surfactants reverse drug resistance, likely via an indirect mechanism. A detailed understanding of how the membrane affects Pgp substrates and Pgp's catalytic cycle may lead to new strategies to combat clinical drug resistance.

New structure-activity relationship studies in a series of N,N-bis(cyclohexanol)amine aryl esters as potent reversers of P-glycoprotein-mediated multidrug resistance (MDR)

As a continuation of previous research on a new series of potent and efficacious P-gp-dependent multidrug resistant (MDR) reversers with a N,N-bis(cyclohexanol)amine scaffold, we have designed and synthesized several analogs by modulation of the two aromatic moieties linked through ester functions to the N,N-bis(cyclohexanol)amine, aiming to optimize activity and to extend structure-activity relationships (SAR) within the series. This scaffold, when esterified with two different aromatic carboxylic acids, gives origin to four geometric isomers (cis/trans, trans/trans, cis/cis and trans/cis). The new compounds were tested on doxorubicin-resistant erythroleukemia K562 cells (K562/DOX) in the pirarubicin uptake assay. Most of them resulted in being potent modulators of the extrusion pump P-gp, showing potency values ([I](0.5)) in the submicromolar and nanomolar range. Of these, compounds 2b, 2c, 3d, 5a-d and 6d, showed excellent efficacy with a α(max) close to 1. Selected compounds (2d, 3a, 3b, 5a-d) were further studied to evaluate their doxorubicin cytotoxicity potentiation (RF) on doxorubicin-resistant erythroleukemia K562 cells and were found able to enhance significantly doxorubicin cytotoxicity on K562/DOX cells. The results of both pirarubicin uptake and the cytotoxicity assay, indicate that the new compounds of the series are potent P-gp-mediated MDR reversers. They present a structure with a mix of flexible and rigid moieties, a property that seems critical to allow the molecules to choose the most productive of the several binding modes possible in the transporter recognition site. In particular, compounds 5c and 5d, similar to the already reported analogous isomers 1c and 1d,(29) are potent and efficacious modulators of P-gp-dependent MDR and may be promising leads for the development of MDR-reversal drugs.

Detection of apoptosis through the lipid order of the outer plasma membrane leaflet

Cell plasma membranes of living cells maintain their asymmetry, so that the outer leaflet presents a large quantity of sphingomyelin, which is critical for formation of ordered lipid domains. Here, a recently developed probe based on Nile Red (NR12S) was applied to monitor changes in the lipid order specifically at the outer leaflet of cell membranes. Important key features of NR12S are its ratiometric response exclusively to lipid order (liquid ordered vs. liquid disordered phase) and not to surface charge, the possibility of using it at very low concentrations (10-20nM) and the very simple staining protocol. Cholesterol extraction, oxidation and sphingomyelin hydrolysis were found to red shift the emission spectrum of NR12S, indicating a decrease in the lipid order at the outer plasma membrane leaflet. Remarkably, apoptosis induced by three different agents (actinomycin D, camptothecin, staurosporine) produced very similar spectroscopic effects, suggesting that apoptosis also significantly decreases the lipid order at this leaflet. The applicability of NR12S to detect apoptosis was further validated by fluorescence microscopy and flow cytometry, using the ratio between the blue and red parts of its emission band. Thus, for the first time, an environment-sensitive probe, sensitive to lipid order, is shown to detect apoptosis, suggesting a new concept in apoptosis sensing.

Inhibition of P-glycoprotein-mediated Multidrug Resistance (MDR) by N,N-bis(cyclohexanol)amine aryl esters: further restriction of molecular flexibility maintains high potency and efficacy

Conformational modulation of the aryl portion of a set of N,N-bis(cyclohexanol)amine aryl esters (1a-d) that are potent Pgp-dependent MDR inhibitors has been performed. Toward this end the trans-3-(3,4,5-trimethoxyphenyl)acrylic acid present in set 1 was substituted with 3-(3,4,5-trimethoxyphenyl)propanoic and 3-(3,4,5-trimethoxyphenyl)propiolic moieties to give sets 2 and 3, respectively. While the introduction of 3-(3,4,5-trimethoxyphenyl)propanoic moiety resulted in a definite drop in potency and efficacy, esterification with 3-(3,4,5-trimethoxyphenyl)propiolic acid gave four isomers (3a-d) that maintain high potency and possess optimal efficacy. These results are discussed in terms of conformational flexibility of the different sets of compounds.

Lipid dependence of ABC transporter localization and function

Lipid rafts have been implicated in many cellular functions, including protein and lipid transport and signal transduction. ATP-binding cassette (ABC) transporters have also been localized in these membrane domains. In this review the evidence for this specific localization will be evaluated and discussed in terms of relevance to ABC transporter function. We will focus on three ABC transporters of the A, B and C subfamily, respectively. Two of these transporters are relevant to multidrug resistance in tumor cells (Pgp/ABCB1 and MRP1/ABCC1), while the third (ABCA1) is extensively studied in relation to the reverse cholesterol pathway and cellular cholesterol homeostasis. We will attempt to derive a generalized model of lipid rafts to which they associate based on the use of various different lipid raft isolation procedures. In the context of lipid rafts, modulation of ABC transporter localization and function by two relevant lipid classes, i.e. sphingolipids and cholesterol, will be discussed.

Depletion of phytosterols from the plant plasma membrane provides evidence for disruption of lipid rafts

Involvement of sterols in membrane structural properties has been extensively studied in model systems but rarely assessed in natural membranes and never investigated for the plant plasma membrane (PM). Here, we address the question of the role of phytosterols in the organization of the plant PM. The sterol composition of tobacco BY-2 cell PM was determined by gas chromatography. The cyclic oligosaccharide methyl-beta-cyclodextrin, commonly used in animal cells to decrease cholesterol levels, caused a drastic reduction (50%) in the PM total free sterol content of the plant material, without modification in amounts of steryl-conjugates. Fluorescence spectroscopy experiments using DPH, TMA-DPH, Laurdan, and di-4-ANEPPDHQ indicated that such a depletion in sterol content increased lipid acyl chain disorder and reduced the overall liquid-phase heterogeneity in correlation with the disruption of phytosterol-rich domains. Methyl-beta-cyclodextrin also prevented isolation of a PM fraction resistant to solubilization by nonionic detergents, previously characterized in tobacco, and induced redistribution of the proteic marker of this fraction, NtrbohD, within the membrane. Altogether, our results support the role of phytosterols in the lateral structuring of the PM of higher plant cells and suggest that they are key compounds for the formation of plant PM microdomains.

Lipid rafts: dream or reality for cholesterol transporters?

As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.

Perturbation of membrane microdomains in GLC4 multidrug-resistant lung cancer cells − modification of ABCC1 (MRP1) localization and functionality

The multidrug resistance-associated protein transporter ABCC1 (MRP1) is an integral plasma membrane protein involved in the multidrug resistance phenotype. It actively expels a number of cytotoxic molecules from cells. To gain insight into the modulation of the functional properties of this integral membrane protein by cholesterol, a main component of the lipid bilayer, we used multidrug-resistant GLC4/ADR cells, which overexpress MRP1. Upon altering the plasma membrane cholesterol content of these cells, membrane localization and the activity of MRP1 were analyzed. A detergent-free methodology was used to separate "light" and "heavy" plasma membrane fractions. Our data show that MRP1 was exclusively found in "light" fractions known as L0 phase membrane microdomains, together with 23% of gangliosides GM1 and 40% of caveolin-1. Depletion of the membrane cholesterol level to 40% by treatment with the cholesterol-chelating agent methyl-beta-cyclodextrin did not modify MRP1 activity, as evidenced either by the rate of efflux of pirarubicin or that of glutathione. Further cholesterol depletion below 40% yielded both a partial shift of MRP1 to the high-density fraction and a decrease of its functionality. Taken together, these data suggest that MRP1 functionality depends on its localization in cholesterol-rich membrane microdomains.

Methyl-β-cyclodextrin enhances the susceptibility of human breast cancer cells to carboplatin and 5-fluorouracil: Involvement of Akt, NF-κB and Bcl-2

The response rates of extensively used chemotherapeutic drugs, carboplatin (Carb) or 5-fluorouracil (5-FU) are relatively disappointing because of considerable side effects associated with their high-dose regimen. In the present study, we determined whether treatment with a cholesterol depleting agent, methyl-beta-cyclodextrin (MCD), enhances the weak efficacy of low doses of Carb or 5-FU in human breast cancer cells. Data demonstrate that pretreatment with MCD significantly potentiates the cytotoxic activity of Carb and 5-FU in both MCF-7 and MDA-MB-231. Furthermore, we explored the molecular basis of enhanced cytotoxicity, and our data revealed that low-dose treatment with these drugs in MCD pretreated cells exhibited significantly decreased Akt phosphorylation, NF-kappaB activity and down-regulation in expression of anti-apoptotic protein Bcl-2. In addition, MCD pretreated cells demonstrated an increased intracellular drug accumulation as compared to cells treated with drugs alone. Taken together, our data provide the basis for potential therapeutic application of MCD in combination with other conventional cytotoxic drugs to facilitate reduction of drug dosage that offers a better chemotherapeutic approach with low toxicity.

Disruption of lipid rafts inhibits P2X1 receptor-mediated currents and arterial vasoconstriction

P2X1 receptors for ATP are ligand-gated cation channels expressed on a range of smooth muscle preparations and blood platelets. The receptors appear to be clustered close to sympathetic nerve varicosities and mediate the underlying membrane potential changes and constriction following nerve stimulation in a range of arteries and resistance arterioles. In this study we have used discontinuous sucrose density gradients, Western blot analysis, and cholesterol measurements to show that recombinant and smooth muscle (rat tail artery, vas deferens, and bladder) P2X1 receptors are present in cholesterol-rich lipid rafts and co-localize with the lipid raft markers flotillin-1 and -2. Lipid rafts are specialized lipid membrane microdomains involved in signaling and trafficking. To determine whether lipid raft association was essential for P2X1 receptor channel function we used the cholesterol-depleting agent methyl-beta-cyclodextrin (10 mm for 1 h). This led to a redistribution of the P2X1 receptor throughout the sucrose gradient and reduced P2X1 receptor-mediated (alpha,beta-methylene ATP, 10 microm) currents in HEK293 cells by >90% and contractions of the rat tail artery by approximately 50%. However contractions evoked by potassium chloride (60 mm) were unaffected by methyl-beta-cyclodextrin and the inactive analogue alpha-cyclodextrin had no effect on P2X1 receptor-mediated currents or contractions. P2X1 receptors are subject to ongoing regulation by receptors and kinases, and the present results suggest that lipid rafts are an essential component in the maintenance of these localized signaling domains and play an important role in P2X1 receptor-mediated control of arteries.