Raft and cytoskeleton associations of an ABC transporter: P-glycoprotein

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.

Cross-Linking Mass Spectrometry on P-Glycoprotein

The ABC transporter P-glycoprotein (Pgp) has been found to be involved in multidrug resistance in tumor cells. Lipids and cholesterol have a pivotal role in Pgp's conformations; however, it is often difficult to investigate it with conventional structural biology techniques. Here, we applied robust approaches coupled with cross-linking mass spectrometry (XL-MS), where the natural lipid environment remains quasi-intact. Two experimental approaches were carried out using different cross-linkers (i) on living cells, followed by membrane preparation and immunoprecipitation enrichment of Pgp, and (ii) on-bead, subsequent to membrane preparation and immunoprecipitation. Pgp-containing complexes were enriched employing extracellular monoclonal anti-Pgp antibodies on magnetic beads, followed by on-bead enzymatic digestion. The LC-MS/MS results revealed mono-links on Pgp's solvent-accessible residues, while intraprotein cross-links confirmed a complex interplay between extracellular, transmembrane, and intracellular segments of the protein, of which several have been reported to be connected to cholesterol. Harnessing the MS results and those of molecular docking, we suggest an epitope for the 15D3 cholesterol-dependent mouse monoclonal antibody. Additionally, enriched neighbors of Pgp prove the strong connection of Pgp to the cytoskeleton and other cholesterol-regulated proteins. These findings suggest that XL-MS may be utilized for protein structure and network analyses in such convoluted systems as membrane proteins.

Modifications of Plasma Membrane Organization in Cancer Cells for Targeted Therapy

Modifications of the composition or organization of the cancer cell membrane seem to be a promising targeted therapy. This approach can significantly enhance drug uptake or intensify the response of cancer cells to chemotherapeutics. There are several methods enabling lipid bilayer modifications, e.g., pharmacological, physical, and mechanical. It is crucial to keep in mind the significance of drug resistance phenomenon, ion channel and specific receptor impact, and lipid bilayer organization in planning the cell membrane-targeted treatment. In this review, strategies based on cell membrane modulation or reorganization are presented as an alternative tool for future therapeutic protocols.

Membrane lipids and transporter function

Transport proteins are essential for cells in allowing the exchange of substances between cells and their environment across the lipid bilayer forming a tight barrier. Membrane lipids modulate the function of transmembrane proteins such as transporters in two ways: Lipids are tightly and specifically bound to transport proteins and in addition they modulate from the bulk of the lipid bilayer the function of transport proteins. This overview summarizes currently available information at the ultrastructural level on lipids tightly bound to transport proteins and the impact of altered bulk membrane lipid composition. Human diseases leading to altered lipid homeostasis will lead to altered membrane lipid composition, which in turn affect the function of transporter proteins.

ω-3 Long Chain Polyunsaturated Fatty Acids as Sensitizing Agents and Multidrug Resistance Revertants in Cancer Therapy

Chemotherapy efficacy is strictly limited by the resistance of cancer cells. The ω-3 long chain polyunsaturated fatty acids (ω-3 LCPUFAs) are considered chemosensitizing agents and revertants of multidrug resistance by pleiotropic, but not still well elucidated, mechanisms. Nowadays, it is accepted that alteration in gene expression, modulation of cellular proliferation and differentiation, induction of apoptosis, generation of reactive oxygen species, and lipid peroxidation are involved in ω-3 LCPUFA chemosensitizing effects. A crucial mechanism in the control of cell drug uptake and efflux is related to ω-3 LCPUFA influence on membrane lipid composition. The incorporation of docosahexaenoic acid in the lipid rafts produces significant changes in their physical-chemical properties affecting content and functions of transmembrane proteins, such as growth factors, receptors and ATP-binding cassette transporters. Of note, ω-3 LCPUFAs often alter the lipid compositions more in chemoresistant cells than in chemosensitive cells, suggesting a potential adjuvant role in the treatment of drug resistant cancers.

Nucleosome stability measured in situ by automated quantitative imaging

Current approaches have limitations in providing insight into the functional properties of particular nucleosomes in their native molecular environment. Here we describe a simple and powerful method involving elution of histones using intercalators or salt, to assess stability features dependent on DNA superhelicity and relying mainly on electrostatic interactions, respectively, and measurement of the fraction of histones remaining chromatin-bound in the individual nuclei using histone type- or posttranslational modification- (PTM-) specific antibodies and automated, quantitative imaging. The method has been validated in H3K4me3 ChIP-seq experiments, by the quantitative assessment of chromatin loop relaxation required for nucleosomal destabilization, and by comparative analyses of the intercalator and salt induced release from the nucleosomes of different histones. The accuracy of the assay allowed us to observe examples of strict association between nucleosome stability and PTMs across cell types, differentiation state and throughout the cell-cycle in close to native chromatin context, and resolve ambiguities regarding the destabilizing effect of H2A.X phosphorylation. The advantages of the in situ measuring scenario are demonstrated via the marked effect of DNA nicking on histone eviction that underscores the powerful potential of topological relaxation in the epigenetic regulation of DNA accessibility.

How plausible is the use of dietary n-3 PUFA in the adjuvant therapy of cancer?

Considerable debate exists regarding the potential antineoplastic effect of dietary long-chain n-3 PUFA contained in fatty fishes. Since the majority of published data has proven that their intake does not induce toxic or carcinogenic effects in humans, their possible preventive use against cancer has been suggested. On the other hand, it is unlikely that they could be effective in cancer patients as a single therapy. Nevertheless, a considerable effort has been put forth in recent years to evaluate the hypothesis that n-3 PUFA might improve the antineoplastic efficiency of currently used anticancer agents. The rationale for this therapeutic combinatory strategy is trying to increase cancer sensitivity to conventional therapies. This could allow the use of lower drug/radiation doses and, thereby, a reduction in the detrimental health effects associated with these treatments. We will here critically examine the studies that have investigated this possibility, by focusing particularly on the biological and molecular mechanisms underlying the antineoplastic effect of these combined treatments. A possible use of n-3 PUFA in combination with the innovative single-targeted anti-cancer therapies, that often are not completely devoid of dangerous side-effects, is also suggested.

Lipid regulation of the ABCB1 and ABCG2 multidrug transporters

This chapter deals with the interactions of two medically important multidrug ABC transporters (MDR-ABC), ABCB1 and ABCG2, with lipid molecules. Both ABCB1 and ABCG2 are capable of transporting a wide range of hydrophobic drugs and xenobiotics and are involved in cancer chemotherapy resistance. Therefore, the exploration of their mechanism of action has major therapeutic consequences. As discussed here in detail, both ABCB1 and ABCG2 are significantly affected by various lipid compounds especially those residing in their close proximity in the plasma membrane. ABCB1 is capable of transporting lipids and lipid derivatives, and thus may alter the general membrane composition by "flopping" membrane lipid constituents, while there is no such information regarding ABCG2. Still, both ABCB1 and ABCG2 show complex interactions with a variety of lipid molecules, and the transporters are significantly modulated by cholesterol and cholesterol derivatives at the posttranslational level. In this chapter, we explore the molecular details of the direct transporter-lipid interactions, the potential role of lipid-sensor domains within the proteins, as well as the application of experimental site-directed mutagenesis, detailed structural studies, and in silico modeling for examining these interactions. We also discuss the regulation of ABCB1 and ABCG2 expression at the transcriptional level, occurring through nuclear receptors involved in lipid sensing. The better understanding of lipid interactions with these medically important MDR-ABC transporters may significantly improve further drug development and clinical treatment options.

miR-103/107 modulates multidrug resistance in human gastric carcinoma by downregulating Cav-1

MicroRNAs (miRNAs) are a class of non-protein-coding small RNAs with the capacity to regulate fundamental biological processes essential for cancer initiation and progression. In the present study, we analyzed miRNA expression levels between multidrug-resistant gastric carcinoma cell line SGC7901/ADR and its parent cell line SGC7901 using a miRNA microarray. MiR-103/107 was downregulated compared with parental SGC7901 cells. Overexpression of miR-103/107 sensitized SGC7901/ADR cells to doxorubicin (DOX), as demonstrated by in vitro and in vivo drug sensitivity assay. We further confirmed that miR-103/107 inhibited P-gp function in gastric cancer SGC7901/ADR cells. Finally, we verified that caveolin-1 (Cav-1), a critical component of lipid rafts, was a target of miR-103/107.

ABCA1, ABCG1, and ABCG4 are distributed to distinct membrane meso-domains and disturb detergent-resistant domains on the plasma membrane

ATP-binding cassette A1 (ABCA1), ABCG1, and ABCG4 are lipid transporters that mediate the efflux of cholesterol from cells. To analyze the characteristics of these lipid transporters, we examined and compared their distributions and lipid efflux activity on the plasma membrane. The efflux of cholesterol mediated by ABCA1 and ABCG1, but not ABCG4, was affected by a reduction of cellular sphingomyelin levels. Detergent solubility and gradient density ultracentrifugation assays indicated that ABCA1, ABCG1, and ABCG4 were distributed to domains that were solubilized by Triton X-100 and Brij 96, resistant to Triton X-100 and Brij 96, and solubilized by Triton X-100 but resistant to Brij 96, respectively. Furthermore, ABCG1, but not ABCG4, was colocalized with flotillin-1 on the plasma membrane. The amounts of cholesterol extracted by methyl-β-cyclodextrin were increased by ABCA1, ABCG1, or ABCG4, suggesting that cholesterol in non-raft domains was increased. Furthermore, ABCG1 and ABCG4 disturbed the localization of caveolin-1 to the detergent-resistant domains and the binding of cholera toxin subunit B to the plasma membrane. These results suggest that ABCA1, ABCG1, and ABCG4 are localized to distinct membrane meso-domains and disturb the meso-domain structures by reorganizing lipids on the plasma membrane; collectively, these observations may explain the different substrate profiles and lipid efflux roles of these transporters.

The ability of hyaluronan fragments to reverse the resistance of C6 rat glioma cell line to temozolomide and carmustine

Aim of the study

Hyaluronan (HA) is an extracellular matrix (ECM) polymer that may contribute to the emergence of anti-cancer drug resistance. Attempts to reverse drug resistance using small hyaluronan oligomers (oHA) are being made. The initial reports suggest that the oHA fraction may effectively reverse anti-cancer drug resistance in glioma models. However, the reversal effects of oHA of defined molecular length on glioma cells have not been investigated yet. In this study, we examined HA fragments containing 2 disaccharide units (oHA-2), 5 disaccharide units (oHA-5), and 68 kDa hyaluronan polymer (HA-68k) as agents possibly reversing the resistance of a C6 rat glioma cell line to temozolomide (TMZ) and carmustine (BCNU).

Material and methods

A 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) viability assay was used to assess the cytotoxicity of TMZ and BCNU in the presence or absence of the hyaluronan fragments. By comparing viability of the cells, the reversal effects of HA fragments on TMZ and BCNU resistance in C6 glioma cells were assessed.

Results

We found statistically significant decreases in the viability of cells in the presence of TMZ+oHA-5 as compared to TMZ alone (51.2 ±4.5 vs. 74.2 ±5.8, p = 0.0031), BCNU+o-HA5 as compared to BCNU alone (49.3 ±4.4 vs. 65.6 ±5.7, p = 0.0119), and BCNU+HA-68k as compared to BCNU alone (55.2 ±2.3 vs. 65.6 ±5.7, p = 0.0496).

Conclusions

Conclusions: Hyaluronan oligomers of 5 disaccharide units (oHA-5) significantly reversed the resistance of C6 cells to TMZ and BCNU. The results are only preliminary and a more thorough follow-up investigation is required to assess their actual role.

Does P-glycoprotein contribute to amphotericin B epithelial transport in Caco-2 cells?

INTRODUCTION

Amphotericin B (AmB) is a highly efficacious therapeutic for invasive fungal infections and protozoal diseases. Increasing prevalence of these conditions warrants the development of an oral AmB formulation. Efflux transporters, such as the ABCB1 gene product P-glycoprotein, affect the oral bioavailability and disposition of a range of clinically relevant compounds. At present, it remains to be determined whether AmB is a substrate of P-glycoprotein mediated efflux. The objective of this study was to determine whether P-glycoprotein contributes to the epithelial transport of AmB in a Caco-2 cell model.

METHODS

Stimulation of P-glycoprotein ATPase activity was assessed using membranes containing human recombinant P-glycoprotein. An ABCB1 knockdown Caco-2 cell model was employed to determine non-toxic concentrations of AmB. AmB cellular association, following a 180 min incubation, was determined using an high performance liquid chromatography-ultraviolet (HPLC-UV) assay.

RESULTS

At the concentrations investigated, AmB did not stimulate P-glycoprotein ATPase activity. Non-toxic concentrations of AmB were 1 μg/mL-5 μg/mL; these were used in subsequent experiments. No significant difference in AmB cellular association was observed for ABCB1 small interfering ribonucleic acid transfected and non-transfected Caco-2 cells, following a 180 min incubation with 1 μg/mL and 2.5 μg/mL AmB. However, significantly greater AmB was associated with transfected cells as compared to non-transfected cells, when cells were incubated with 5 μg/mL AmB.

CONCLUSIONS

These results suggest that AmB is not a substrate of P-glycoprotein mediated efflux in this Caco-2 cell model. P-glycoprotein is not expected to be a major barrier to the oral absorption and disposition of AmB.

The Role of the Actin Cytoskeleton and Lipid Rafts in the Localization and Function of the ABCC1 Transporter

ATP-binding cassette (ABC) transporters are known to be important factors in multidrug resistance of tumor cells. Lipid rafts have been implicated in their localization in the plasma membrane, where they function as drug efflux pumps. This specific localization in rafts may support the activity of ABC/Abc transporters. This raises questions regarding the nature and composition of the lipid rafts that harbor ABC/Abc transporters and the dependence of ABC/Abc transporters—concerning their localization and activity—on lipid raft constituents. Here we review our work of the past 10 years aimed at evaluating whether ABC/Abc transporters are dependent on a particular membrane environment for their function. What is the nature of this membrane environment and which of the lipid raft constituents are important for this dependency? It turns out that cortical actin is of major importance for stabilizing the localization and function of the ABC/Abc transporter, provided it is localized in an actin-dependent subtype of lipid rafts, as is the case for human ABCC1/multidrug resistance-related protein 1 (MRP1) and rodent Abcc1/Mrp1 but not human ABCB1/P-glycoprotein (PGP). On the other hand, sphingolipids do not appear to be modulators of ABCC1/MRP1 (or Abcc1/Mrp1), even though they are coregulated during drug resistance development.

The productive entry pathway of HIV-1 in macrophages is dependent on endocytosis through lipid rafts containing CD4

Macrophages constitute an important reservoir of HIV-1 infection, yet HIV-1 entry into these cells is poorly understood due to the difficulty in genetically manipulating primary macrophages. We developed an effective genetic approach to manipulate the sub-cellular distribution of CD4 in macrophages, and investigated how this affects the HIV-1 entry pathway. Pluripotent Stem Cells (PSC) were transduced with lentiviral vectors designed to manipulate CD4 location and were then differentiated into genetically modified macrophages. HIV-1 infection of these cells was assessed by performing assays that measure critical steps of the HIV-1 lifecycle (fusion, reverse transcription, and expression from HIV-1 integrants). Expression of LCK (which tethers CD4 to the surface of T cells, but is not normally expressed in macrophages) in PSC-macrophages effectively tethered CD4 at the cell surface, reducing its normal endocytic recycling route, and increasing surface CD4 expression 3-fold. This led to a significant increase in HIV-1 fusion and reverse transcription, but productive HIV-1 infection efficiency (as determined by reporter expression from DNA integrants) was unaffected. This implies that surface-tethering of CD4 sequesters HIV-1 into a pathway that is unproductive in macrophages. Secondly, to investigate the importance of lipid rafts (as detergent resistant membranes - DRM) in HIV-1 infection, we generated genetically modified PSC-macrophages that express CD4 mutants known to be excluded from DRM. These macrophages were significantly less able to support HIV-1 fusion, reverse-transcription and integration than engineered controls. Overall, these results support a model in which productive infection by HIV-1 in macrophages occurs via a CD4-raft-dependent endocytic uptake pathway.

Fluorescently labeled methyl-beta-cyclodextrin enters intestinal epithelial Caco-2 cells by fluid-phase endocytosis

Cyclodextrins are widely used excipients for increasing the bioavailability of poorly water-soluble drugs. Their effect on drug absorption in the gastrointestinal tract is explained by their solubility- and permeability-enhancement. The aims of this study were to investigate penetration properties of fluorescently labeled randomly methylated-beta-cyclodextrin (FITC-RAMEB) on Caco-2 cell layer and examine the cellular entry of cyclodextrins on intestinal cells. The permeability of FITC-RAMEB through Caco-2 monolayers was very limited. Using this compound in 0.05 mM concentration the permeability coefficient was 3.35±1.29×10(-8) cm/s and its permeability did not change in the presence of 5 mM randomly methylated-beta-cyclodextrin. Despite of the low permeability, cellular accumulation of FITC-RAMEB in cytoplasmic vesicles was significant and showed strong time and concentration dependence, similar to the characteristics of the macropinocytosis marker Lucifer Yellow. The internalization process was fully inhibited at 0°C and it was drastically reduced at 37°C applying rottlerin, an inhibitor of macropinocytosis. Notably, FITC-RAMEB colocalized with the early endosome organizer Rab5a. These results have revealed that FITC-RAMEB is able to enter intestinal epithelial cells by fluid-phase endocytosis from the apical side. This mechanism can be an additional process which helps to overcome the intestinal barrier and contributes to the bioavailability enhancement of cyclodextrins.

Omega 3 fatty acids chemosensitize multidrug resistant colon cancer cells by down-regulating cholesterol synthesis and altering detergent resistant membranes composition

Background

The activity of P-glycoprotein (Pgp) and multidrug resistance related protein 1 (MRP1), two membrane transporters involved in multidrug resistance of colon cancer, is increased by high amounts of cholesterol in plasma membrane and detergent resistant membranes (DRMs). It has never been investigated whether omega 3 polyunsatured fatty acids (PUFAs), which modulate cholesterol homeostasis in dyslipidemic syndromes and have chemopreventive effects in colon cancer, may affect the response to chemotherapy in multidrug resistant (MDR) tumors.

Methods

We studied the effect of omega 3 PUFAs docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) in human chemosensitive colon cancer HT29 cells and in their MDR counterpart, HT29-dx cells.

Results

MDR cells, which overexpressed Pgp and MRP1, had a dysregulated cholesterol metabolism, due to the lower expression of ubiquitin E3 ligase Trc8: this produced lower ubiquitination rate of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCoAR), higher cholesterol synthesis, higher cholesterol content in MDR cells. We found that DHA and EPA re-activated Trc8 E3 ligase in MDR cells, restored the ubiquitination rate of HMGCoAR to levels comparable with chemosensitive cells, reduced the cholesterol synthesis and incorporation in DRMs. Omega 3 PUFAs were incorporated in whole lipids as well as in DRMs of MDR cells, and altered the lipid composition of these compartments. They reduced the amount of Pgp and MRP1 contained in DRMs, decreased the transporters activity, restored the antitumor effects of different chemotherapeutic drugs, restored a proper tumor-immune system recognition in response to chemotherapy in MDR cells.

Conclusions

Our work describes a new biochemical effect of omega 3 PUFAs, which can be useful to overcome chemoresistance in MDR colon cancer cells.

P-glycoprotein-dependent resistance of cancer cells toward the extrinsic TRAIL apoptosis signaling pathway

The TNF-related apoptosis-inducing ligand (TRAIL or Apo2L) preferentially cause apoptosis of malignant cells in vitro and in vivo without severe toxicity. Therefore, TRAIL or agonist antibodies to the TRAIL DR4 and DR5 receptors are used in cancer therapy. However, many malignant cells are intrinsically resistant or acquire resistance to TRAIL. It has been previously proposed that the multidrug transporter P-glycoprotein (Pgp) might play a role in resistance of cells to intrinsic apoptotic pathways by interfering with components of ceramide metabolism or by modulating the electrochemical gradient across the plasma membrane. In this study we investigated whether Pgp also confers resistance toward extrinsic death ligands of the TNF family. To this end we focused our study on HeLa cells carrying a tetracycline-repressible plasmid system which shuts down Pgp expression in the presence of tetracycline. Our findings demonstrate that expression of Pgp is a significant factor conferring resistance to TRAIL administration, but not to other death ligands such as TNF-α and Fas ligand. Moreover, blocking Pgp transport activity sensitizes the malignant cells toward TRAIL. Therefore, Pgp transport function is required to confer resistance to TRAIL. Although the resistance to TRAIL-induced apoptosis is Pgp specific, TRAIL itself is not a direct substrate of Pgp. Pgp expression has no effect on the level of the TRAIL receptors DR4 and DR5. These findings might have clinical implications since the combination of TRAIL therapy with administration of Pgp modulators might sensitize TRAIL resistant tumors.

Single molecule tracking of P-glycoprotein in live cells reveals dynamic heterogeneity

P-glycoprotein transports chemotherapy drugs from the plasma membrane and allows cancer cells to survive treatment. We transiently transfected PGP labeled with enhanced green fluorescent protein (PGP-EGFP) into MES-SA cells and used single molecule tracking techniques to characterize the dynamics on the surface of live cells. PGP exhibits freely diffusive behavior at short times and is confined at long times with a transition to anomalous diffusion at 0.7 s.

Role of CD44 in tumour progression and strategies for targeting

CD44 or hyaluronan receptor is a transmembrane receptor associated with aggressive tumour growth, proliferation, and metastasis. In normal physiology, this receptor has a crucial role in cell adhesion, inflammation, and repair processes. However, many tumour cells over-express this receptor and abuse it to become progressive and perpetual units. The article comments from common functioning of the CD44 receptor, to its diabolic multi-dimensional effects in promotion of malignant cells. It also illuminates the relations of CD44 endorsed processes with other biomolecular events in cancer progression. In an end, the review focuses comprehensively at ongoing researches to exploit the CD44 over-expression as a probable target in treatment, management, and diagnosis of malignancy.

The function of the ATP-binding cassette (ABC) transporter ABCB1 is not susceptible to actin disruption

Previously we have shown that the activity of the multidrug transporter ABCC1 (multidrug resistance protein 1), and its localization in lipid rafts, depends on cortical actin (Hummel I, Klappe K, Ercan C, Kok JW. Mol. Pharm. 2011 79, 229-40). Here we show that the efflux activity of the ATP-binding cassette (ABC) family member ABCB1 (P-glycoprotein), did not depend on actin, neither in ABCB1 over expressing murine National Institutes of Health (NIH) 3T3 MDR1 G185 cells nor in human SK-N-FI cells, which endogenously express ABCB1. Disruption of the actin cytoskeleton, upon treatment of the cells with latrunculin B or cytochalasin D, caused severe changes in cell and membrane morphology, and concomitant changes in the subcellular distribution of ABCB1, as revealed by confocal laser scanning and electron microscopy. Nevertheless, irrespective of actin perturbation, the cell surface pool of ABCB1 remained unaltered. In NIH 3T3 MDR1 G185 cells, ABCB1 is partly localized in detergent-free lipid rafts, which partitioned in two different density gradient regions, both enriched in cholesterol and sphingolipids. Interestingly, disruption of the actin cytoskeleton did not change the density gradient distribution of ABCB1. Our data demonstrate that the functioning of ABCB1 as an efflux pump does not depend on actin, which is due to its distribution in both cell surface-localized non-raft membrane areas and lipid raft domains, which do not depend on actin stabilization.

Doxorubicin and MBO-asGCS oligonucleotide loaded lipid nanoparticles overcome multidrug resistance in adriamycin resistant ovarian cancer cells (NCI/ADR-RES)

The objective of this study was to increase the potency of doxorubicin against adriamycin-resistant NCI/ADR-RES cells by concurrent treatment with doxorubicin and MBO-asGCS loaded solid-lipid nanoparticles (SLN). Loading doxorubicin as ion-pair complex with deoxytaurocholate into cationic and neutral SLN was investigated. Fast release and poor entrapment were observed in cationic nanoparticles, which were corrected by entrapping the complex in neutral polyoxyethylene (20) stearyl ether (Brij(®) 78)/VitE-TPGS nanoparticles. Slow doxorubicin release confirmed the influence of charge and electrolytes on the dissociation of ion-pair complexes. To evaluate antitumor activity, NCI/ADR-RES cells were treated with separate SLN: one loaded with doxorubicin and another carrying MBO-asGCS oligonucleotide. The viability of cells treated with 5 μM doxorubicin was reduced to 17.2% whereas viability was reduced to 2.5% for cells treated with both 5 μM doxorubicin SLN and 100 nM MBO-asGCS SLN. This suggested enhanced apoptosis due to sensitization and effective intracellular delivery of MBO-asGCS and doxorubicin by SLN.

Single-vesicle estimation of ATP-binding cassette transporters in microfluidic channels

We have developed a method to analyze the substrate transport of ATP-binding cassette (ABC) transporters, which are associated with drug resistance in tumor cells. Our microfluidic method is well suited to the single-vesicle estimation of substrate transport and the rapid drug screening of ABC transporters. Using this method, we have demonstrated, for the first time, the analysis of substrate transport by a single transporter and performed drug-inhibition experiments in less than 3 h.

Genome integrity, stem cells and hyaluronan

Faithful preservation of genome integrity is the critical mission of stem cells as well as of germ cells. Reviewed are the following mechanisms involved in protecting DNA in these cells: (a) The efflux machinery that can pump out variety of genotoxins in ATP-dependent manner; (b) the mechanisms maintaining minimal metabolic activity which reduces generation of reactive oxidants, by-products of aerobic respiration; (c) the role of hypoxic niche of stem cells providing a gradient of variable oxygen tension; (d)(e) the presence of hyaluronan (HA) and HA receptors on stem cells and in the niche; (f) the role of role of HA in protecting DNA from oxidative damage; (g) the specific role of HA that may play a role protecting DNA in stem cells; (h) the interactions of HA with sperm cells and oocytes that also may shield their DNA from oxidative damage, and (e) mechanisms by which HA exerts the anti-oxidant activity. While HA has multitude of functions its anti-oxidant capabilities are often overlooked but may be of significance in preservation of integrity of stem and germ cells genome.

Pharmacogenomic determination of genes associated with sensitivity or resistance of tumor cells to curcumin and curcumin derivatives

Curcuma longa L. has long been used as a medicinal plant in traditional Chinese medicine against abdominal disorders. Its active constituent curcumin has anti-inflammatory, chemopreventive and cytotoxic properties. In the present investigation, we have analyzed the cytotoxic activity of curcumin and four derivatives. Among these compounds, ethoxycurcumintrithiadiazolaminomethylcarbonate was the most cytotoxic one. The curcumin-type compounds were not cross-resistant to standard anticancer drugs and were not involved in ATP-binding cassette transporter-mediated multidrug resistance. A combined approach of messenger RNA-based microarray profiling, COMPARE analyses and signaling pathway analyses identified genes as determinants of sensitivity and resistance to curcumin and specific signaling routes involved in cellular response to curcumin. These genes may be useful as biomarkers to develop individualized treatment options in the future. From a nutritional point of view, it is a thriving perspective to further investigate whether C. longa may be used as a spice to improve cancer therapy.

Multidrug resistance-related protein 1 (MRP1) function and localization depend on cortical actin

MRP1 (ABCC1) is known to be localized in lipid rafts. Here we show in two different cell lines that localization of Mrp1/MRP1 (Abcc1/ABCC1) in lipid rafts and its function as an efflux pump are dependent on cortical actin. Latrunculin B disrupts both cortical actin and actin stress fibers. This results in partial loss of actin and Mrp1/MRP1 (Abcc1/ABCC1) from detergent-free lipid raft fractions, partial internalization of Mrp1/MRP1 (Abcc1/ABCC1), and reduction of Mrp1/MRP1 (Abcc1/ABCC1)-mediated efflux. Pretreatment with nocodazole prevents latrunculin B-induced loss of cortical actin and all effects of latrunculin B on Mrp1 (Abcc1) localization and activity. However, pretreatment with tyrphostin A23 does not prevent latrunculin B-induced loss of cortical actin, lipid raft association, and efflux activity, but it does prevent latrunculin B-induced internalization of Mrp1 (Abcc1). Cytochalasin D disrupts actin stress fibers but not cortical actin and this inhibitor much less affects Mrp1/MRP1 (Abcc1/ABCC1) localization in lipid rafts, internalization, and efflux activity. In conclusion, cortical actin disruption results in reduced Mrp1/MRP1 (Abcc1/ABCC1) activity concomitant with a partial shift of Mrp1/MRP1 (Abcc1/ABCC1) out of lipid raft fractions and partial internalization of the ABC transporter. The results suggest that reduced Mrp1 (Abcc1) function is correlated to the loss of lipid raft association but not internalization of Mrp1 (Abcc1).

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

The effects of detergents on the polymerization properties of actin

Effects of some detergents-most frequently used in membrane raft studies-on the polymerization properties of actin were examined under in vitro and in vivo conditions, for protein and cellular investigations, respectively. Under in vitro conditions the polymerization rates were measured with pyrene-labeled actin. We found that polymerization rate depended on the detergent concentration by following either biphasic characteristics or only decreasing tendency. The strongest effects were observed at relatively low detergent concentrations. SDS-PAGE electrophoresis and dynamic light-scattering measurements provided further evidences for the size distribution of actin filaments formed under the influence of detergents. Comparing the polymerization rates measured in the presence of different detergents to those obtained with various magnesium and KCl concentrations showed that detergents may influence the actin polymerization at three levels by modifying: (i) the monomer-monomer interaction, (ii) the local ionic strength, and (iii) the affinity of actin for various cations. In vivo studies on NIH 3T3MDR1 cells using TRITC-phalloidin detected fast depolymerization of large extent around the critical micellar concentrations of the detergents. We concluded that microdomain insolubility observed in the presence of detergents is hardly to be the result of the stabilization of the submembrane actin cytoskeleton merely; rather inter-lipid and lipid-protein interactions are also involved within the detergent-resistant membranes.

Drug resistance is affected by colocalization of P-glycoproteins in raft-like structures unexpected in eggshells of the nematode Haemonchus contortus

In nematodes as in other eukaryotes, there is increasing evidence that drug resistance depends on both changes in the drug cellular targets and in nonspecific mechanisms, involving cellular detoxification by efflux pumps. In vertebrates, P-glycoproteins (Pgp) are membrane efflux pumps responsible for the elimination of xenobiotic agents, especially drugs. We previously reported the presence of Pgp pumps in eggshells and cuticles of the nematode Haemonchus contortus. Eggshells and cuticles are different from cell membranes, in particular they include a chitin layer. Nevertheless these structures present some common biological features with cell membranes and play a role in xenobiotic transport. Pgp activity has been shown to depend on the lipid environment and, in particular, on the cholesterol content in both vertebrate and nematode models. In vertebrates, Pgp is in part located in membrane cholesterol-enriched microdomains, the rafts. We describe here, for the first time, lipid microdomains in eggshells that could correspond with raft-like structures (RLSs). Moreover, a large proportion of the Pgp was shown to colocalize with these RLSs. The functional consequences of the colocalization for xenobiotic transport and thus drug resistance in nematodes were analyzed and compared with results obtained in vertebrates. An understanding of such mechanisms is crucial in overcoming the failure of drug treatments due to the development of resistance.

Hyaluronan induces migration of multidrug-resistant lymphoma cell lines in vitro through Tiam1 activation by a PI3K-dependent mechanism

Hyaluronan (HA) modulates multidrug resistance (MDR) as well as cell migration. Tiam1 is involved in cytoskeleton reorganization during tumor invasion. In this report we show the relationship among HA, Tiam1, migration and MDR in murine lymphoma cell lines. We observed that MDR cells presented higher migratory capacity towards HA in vitro as well as higher constitutive active Tiam1 expression than the sensitive cell line. Besides, HA treatment induced migration towards HA of MDR cell lines through Tiam1 activation by a PI3K-dependent mechanism, showing that disruption of HA signaling would be useful in treatment of MDR hematological malignancies.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Model system for the analysis of cell surface expression of human ABCA1

BACKGROUND

The ABCA1 protein plays a pivotal role in reverse cholesterol transport, by mediating the generation of HDL particles and removing cellular cholesterol. Both the proper expression of ABCA1 in the plasma membrane and the internalization along with apoA-I are required for function. Therefore, we developed a model system to investigate the effect of clinically relevant drugs on the cell surface appearance of ABCA1.

RESULTS

By retroviral transduction system, we established stable mammalian cell lines expressing functional and non-functional ABCA1 variants, tagged with an extracellular hemagglutinin epitope. After characterization of the expression, proper localization and function of different ABCA1 variants, we followed quantitatively their cell surface expression by immunofluorescent staining, using flow cytometry. As expected, we found increased cell surface expression of ABCA1 after treatment with a calpain inhibitor, and observed a strong decrease in plasma membrane ABCA1 expression upon treatment with a trans-Golgi transport inhibitor, Brefeldin A. We tested cholesterol level lowering drugs and other potential inhibitors of ABCA1. Here we demonstrate that ezetimibe affects ABCA1 cell surface expression only in the case of a functional ABCA1.

CONCLUSIONS

Our model system allows a quantitative detection of cell surface expression of ABCA1, screening of substrates or specific inhibitors, and investigating transport regulation.

Inhibition of Functional Hyaluronan-CD44 Interactions in CD133-positive Primary Human Ovarian Carcinoma Cells by Small Hyaluronan Oligosaccharides

PURPOSE: CD44 is one of the most common markers used for identification of highly tumorigenic subpopulations of human carcinoma cells, but little is known about the function of CD44 or its major ligand, hyaluronan, in these cells. The purpose of this study was to investigate the involvement of hyaluronan and its interaction with CD44 in the properties of a tumorigenic subpopulation of primary ovarian carcinoma cells. EXPERIMENTAL DESIGN: A tumorigenic subpopulation was identified in ascites fluids from ovarian carcinoma patients by expression of high CD133 levels. Treatment with small hyaluronan oligosaccharides, which dissociate constitutive hyaluronan polymer-CD44 interactions, was used to test the importance of hyaluronan-CD44 interaction in assembly of multidrug and monocarboxylate transporters and receptor tyrosine kinases in the plasma membrane of cells with high CD133 levels, and in the tumorigenic capacity of the CD133-high subpopulation. RESULTS: Although total CD44 levels were similar in cells with high or low CD133 expression, CD44 was present in close association with transporters, receptor tyrosine kinases, and emmprin (CD147) in the plasma membrane of cells with high CD133 levels. Treatment with small hyaluronan oligosaccharides reduced association of the transporters and receptor tyrosine kinases with CD44 in the plasma membrane, diminished drug transporter activity, and inhibited i.p. tumorigenesis in these cells. CONCLUSIONS: We conclude that hyaluronan-CD44 interaction plays an important role in the properties of highly tumorigenic cells by stabilizing oncogenic complexes in their plasma membrane, and that treatment with hyaluronan-CD44 antagonists provides a logical therapeutic approach for abrogating the properties of these cells. (Clin Cancer Res 2009;15(24):7593-601).

Effect of membrane cholesterol on BSEP/Bsep activity: species specificity studies for substrates and inhibitors

The efflux transporter responsible for the canalicular elimination of bile salts from the hepatocytes is the bile salt export pump (BSEP, ABCB11). Absence or inhibition of this transporter leads to bile salt retention in the hepatocyte and in turn can lead to cholestatic liver disease. We expressed the BSEP/Bsep protein from three species (human, rat, and mouse) in a baculovirus-infected Sf9 system. Vesicles prepared from these cells were used to evaluate bile salt transport of four conjugated bile salts. Because the Sf9 system contains less membrane cholesterol than the liver canalicular membrane, the effect of added cholesterol on the kinetics of BSEP/Bsep-mediated bile salt transport was also investigated. Cholesterol treatment increased the V(max) values in all the species, with the most pronounced effect observed in the rat transporter. In contrast, K(m) values, with the exception of glycochenodeoxycholate, remained largely unchanged. The species-specific bile salt transport inhibition potential of three compounds known to cause clinical cholestasis was investigated in vesicles containing BSEP/Bsep. Troglitazone and glibenclamide inhibited the BSEP/Bsep-mediated transport of different bile salts with similar affinities, whereas the potential of cyclosporine A to inhibit bile salt transport showed species- and bile salt-specific variations. In conclusion, the cholesterol-loaded Sf9 vesicles overexpressing BSEP/Bsep seem to be a useful system for the identification of potential cholestatic compounds and can also be used for the investigation of species specificity. We observed greater differences in IC(50) values for inhibitors than in K(m) values for substrates between species.

Abrogating drug resistance in malignant peripheral nerve sheath tumors by disrupting hyaluronan-CD44 interactions with small hyaluronan oligosaccharides

Malignant peripheral nerve sheath tumors (MPNST) develop in approximately 10% of neurofibromatosis type-1 patients and are a major contributing factor to neurofibromatosis-1 patient mortality and morbidity. MPNSTs are multidrug resistant, and thus long-term patient survival rates are poor after standard doxorubicin or multiagent chemotherapies. We show that the hyaluronan receptor CD44 forms complexes with multidrug transporters, BCRP (ABCG2) and P-glycoprotein (ABCB1), in the plasma membrane of human MPNST cells. Small hyaluronan oligosaccharides antagonize hyaluronan-CD44-mediated processes and inhibit hyaluronan production. Treatment of MPNST cells with the hyaluronan oligomers causes disassembly of CD44-transporter complexes and induces internalization of CD44, BCRP, and P-glycoprotein. Consequently, the oligomers suppress drug transporter activity and increase sensitivity to doxorubicin treatment in culture. In vivo, systemic administration of hyaluronan oligomers inhibits growth of MPNST xenografts. Moreover, the oligomers and doxorubicin act synergistically in vivo, in that combined suboptimal doses induce tumor regression to a greater extent than the additive effects of each agent alone. These findings indicate that constitutive hyaluronan-CD44 interactions contribute to drug transporter localization and function at the plasma membrane, and that attenuating hyaluronan-CD44 interactions sensitizes MPNSTs to doxorubicin in vitro and in vivo. These results also show the potential efficacy of hyaluronan oligomers, which are nontoxic and nonimmunogenic, as an adjuvant for chemotherapy in MPNST patients.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

High heterogeneity of plasma membrane microfluidity in multidrug-resistant cancer cells

Diffusion-time distribution analysis (DDA) has been used to explore the plasma membrane fluidity of multidrug-resistant cancer cells (LR73 carcinoma cells) and also to characterize the influence of various membrane agents present in the extracellular medium. DDA is a recent single-molecule technique, based on fluorescence correlation spectroscopy (FCS), well suited to retrieve local organization of cell membrane. The method was conducted on a large number of living cells, which enabled us to get a detailed overview of plasma membrane microviscosity, and plasma membrane micro-organization, between the cells of the same line. Thus, we clearly reveal the higher heterogeneity of plasma membrane in multidrug-resistant cancer cells in comparison with the nonresistant ones (denoted sensitive cells). We also display distinct modifications related to a membrane fluidity modulator, benzyl alcohol, and two revertants of multidrug resistance, verapamil and cyclosporin-A. A relation between the distribution of the diffusion-time values and the modification of membrane lateral heterogeneities is proposed.

Mouse Bsep ATPase Assay: A Nonradioactive Tool for Assessment of the Cholestatic Potential of Drugs

The mouse ortholog of the human bile salt export pump (BSEP) transporter was expressed in a baculovirus-infected insect cell (Sf9) system to study the effect of membrane cholesterol content on the transporter function. The transport activity of cholesterol-loaded mouse Bsep-HAM-Sf9 vesicles was determined in a vesicular transport assay with taurochenodeoxycholate (TCDC), a known BSEP substrate. Mouse Bsep transports TCDC at a high rate that can be sensitively detected in the ATPase assay. Cholesterol upload of the Sf9 membrane potentiates both TCDC transport and TCDC-stimulated ATPase activities. Inhibitory effect of BSEP interactors on probe substrate transport was tested in both vesicular transport and ATPase assays using cholesterol-loaded membrane vesicles. A good rank order correlation was found between IC50 values measured in TCDC-stimulated mBsep ATPase assay and in the human BSEP vesicular transport assay utilizing taurocholate (TC) as probe substrate. This upgraded form of the mouse Bsep-HAM ATPase assay is a user friendly, sensitive, nonradioactive method for early high-throughput screening of drugs with BSEP-related cholestatic potential. It may complement the human BSEP-mediated taurocholate vesicular transport inhibition assay. ( Journal of Biomolecular Screening 2009:10-15)

Membrane homoeostasis and multidrug resistance in yeast

The development of MDR (multidrug resistance) in yeast is due to a number of mechanisms. The most documented mechanism is enhanced extrusion of drugs mediated by efflux pump proteins belonging to either the ABC (ATP-binding cassette) superfamily or MFS (major facilitator superfamily). These drug-efflux pump proteins are localized on the plasma membrane, and the milieu therein affects their proper functioning. Several recent studies demonstrate that fluctuations in membrane lipid composition affect the localization and proper functioning of the MDR efflux pump proteins. Interestingly, the efflux pumps of the ABC superfamily are particularly susceptible to imbalances in membrane-raft lipid constituents. This review focuses on the importance of the membrane environment in functioning of the drug-efflux pumps and explores a correlation between MDR and membrane lipid homoeostasis.

Cytometry of raft and caveola membrane microdomains: from flow and imaging techniques to high throughput screening assays

The evolutionarily developed microdomain structure of biological membranes has gained more and more attention in the past decade. The caveolin-free "membrane rafts," the caveolin-expressing rafts (caveolae), as well as other membrane microdomains seem to play an essential role in controlling and coordinating cell-surface molecular recognition, internalization/endocytosis of the bound molecules or pathogenic organisms and in regulation of transmembrane signal transduction processes. Therefore, in many research fields (e.g. neurobiology and immunology), there is an ongoing need to understand the nature of these microdomains and to quantitatively characterize their lipid and protein composition under various physiological and pathological conditions. Flow and image cytometry offer many sophisticated and routine tools to study these questions. In this review, we give an overview of the past efforts to detect and characterize these membrane microdomains by the use of classical cytometric technologies, and finally we will discuss the results and perspectives of a new line of raft cytometry, the "high throughput screening assays of membrane microdomains," based on "lipidomic" and "proteomic" approaches.

Hyaluronan-CD44 interaction activates stem cell marker Nanog, Stat-3-mediated MDR1 gene expression, and ankyrin-regulated multidrug efflux in breast and ovarian tumor cells

Hyaluronan (HA) is a major glycosaminoglycan in the extracellular matrix whose expression is tightly linked to multidrug resistance and tumor progression. In this study we investigated HA-induced interaction between CD44 (a HA receptor) and Nanog (an embryonic stem cell transcription factor) in both human breast tumor cells (MCF-7 cells) and human ovarian tumor cells (SK-OV-3.ipl cells). Using a specific primer pair to amplify Nanog by reverse transcriptase-PCR, we detected the expression of Nanog transcript in both tumor cell lines. In addition, our results reveal that HA binding to these tumor cells promotes Nanog protein association with CD44 followed by Nanog activation and the expression of pluripotent stem cell regulators (e.g. Rex1 and Sox2). Nanog also forms a complex with the "signal transducer and activator of transcription protein 3" (Stat-3) in the nucleus leading to Stat-3-specific transcriptional activation and multidrug transporter, MDR1 (P-glycoprotein) gene expression. Furthermore, we observed that HA-CD44 interaction induces ankyrin (a cytoskeletal protein) binding to MDR1 resulting in the efflux of chemotherapeutic drugs (e.g. doxorubicin and paclitaxel (Taxol)) and chemoresistance in these tumor cells. Overexpression of Nanog by transfecting tumor cells with Nanog cDNA stimulates Stat-3 transcriptional activation, MDR1 overexpression, and multidrug resistance. Down regulation of Nanog signaling or ankyrin function (by transfecting tumor cells with Nanog small interfering RNA or ankyrin repeat domain cDNA) not only blocks HA/CD44-mediated tumor cell behaviors but also enhances chemosensitivity. Taken together, these findings suggest that targeting HA/CD44-mediated Nanog-Stat-3 signaling pathways and ankyrin/cytoskeleton function may represent a novel approach to overcome chemotherapy resistance in some breast and ovarian tumor cells displaying stem cell marker properties during tumor progression.

Salmonella enterica serovar Typhimurium modulates P-glycoprotein in the intestinal epithelium

Studies over the last decade have shown that Salmonella enterica serovar Typhimurium (S. typhimurium) is able to preferentially locate to sites of tumor growth and modulate (shrink) the growth of many cancers. Given this unique association between S. typhimurium and cancer cells, the objective of this study was to investigate the capacity of this microorganism to modulate the plasma membrane multidrug resistance (MDR) protein P-glycoprotein (P-gp), an ATP-binding cassette transporter responsible for effluxing many cancer drugs. Using an in vitro model of S. typhimurium infection of polarized human cancer intestinal cell lines, we have found that this enteric pathogen functionally downregulates the efflux capabilities of P-gp. Specifically, we show that S. typhimurium infection of human intestinal cancer cells results in the enhanced intracellular accumulation of a number of P-gp substrates that corresponds to the posttranscriptional downregulation of P-gp expression. Furthermore, cells expressing small interfering RNAs against MDR1, the gene encoding P-gp, were significantly more susceptible to the cytotoxic effects of bacterial infection. This result is consistent with our observation that S. typhimurium was significantly less able to invade cells overexpressing MDR1. Taken together, these results reveal a novel role for P-gp in the maintenance of homeostasis in the gastrointestinal tract in regard to bacterial infection. Thus the regulation of P-gp by S. typhimurium has important implications not only for the development of new cancer therapeutics aimed at reversing drug resistance but also in the understanding of how microbes have evolved diverse strategies to interact with their host.

Hyaluronan, CD44 and Emmprin: partners in cancer cell chemoresistance

Hyaluronan not only is an important structural component of extracellular matrices but also interacts with cells during dynamic cell processes such as those occurring in cancer. Consequently, interactions of hyaluronan with tumor cells play important cooperative roles in various aspects of malignancy. Hyaluronan binds to several cell surface receptors, including CD44, thus leading to co-regulation of signaling pathways that are important in regulation of multidrug resistance to anticancer drugs, in particular anti-apoptotic pathways induced by activation of receptor tyrosine kinases. Emmprin, a cell surface glycoprotein of the Ig superfamily, stimulates hyaluronan production and downstream signaling consequences. Emmprin and CD44 also interact with various multidrug transporters of the ABC family and monocarboxylate transporters associated with resistance to cancer therapies. Moreover, hyaluronan-CD44 interactions are critical to these properties in the highly malignant, chemotherapy-resistant cancer stem-like cells. Perturbations of the hyaluronan-CD44 interaction at the plasma membrane by various antagonists result in attenuation of receptor tyrosine kinase and transporter activities and inhibition of tumor progression in vivo. These antagonists, especially small hyaluronan oligomers, may be useful in therapeutic strategies aimed at preventing tumor refractoriness or recurrence due to drug-resistant sub-populations within malignant cancers.

P-glycoprotein inhibition by membrane cholesterol modulation

P-glycoprotein (Pgp) is a transmembrane protein that actively exports lipophilic chemotherapeutics from the cells causing multidrug resistance. Pgp molecules are partially localized in TX-100-resistant rafts, and the activity of the transporter is highly sensitive to the presence of cholesterol. To better understand these relationships, the influence of membrane cholesterol content on Pgp function, as measured via calcein accumulation, was studied in correlation with changes elicited in membrane structure. Membrane cholesterol was modulated by heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) and the cholesterol inclusion complex of DIMEB (Chol-DIMEB). Changes in membrane cholesterol level were reflected by alterations in the overall lipid packing as measured by Merocyanine 540 (MC540) staining and were also accompanied by changes in the raft association of the pump. DIMEB and Chol-DIMEB treatments have also lead to increased permeability of the cell membrane in both directions, raising the possibility that the effects on pumping efficiency reflect leakage of ATP also from the non-permeabilized cells. However, the treatments did not influence the intracellular ATP levels of the non-permeabilized cells. Our data suggest that Pgp inhibition by cyclodextrin treatments arises through modulation of its membrane microenvironment, rather than as a result of concomitant cytotoxicity.