Drug resistance-associated changes in sphingolipids and ABC transporters occur in different regions of membrane domains

Comparative lipidomics of 5-Fluorouracil-sensitive and -resistant colorectal cancer cells reveals altered sphingomyelin and ceramide controlled by acid sphingomyelinase (SMPD1)

5-Fluorouracil (5-FU) is a chemotherapeutic drug widely used to treat colorectal cancer. 5-FU is known to gradually lose its efficacy in treating colorectal cancer following the acquisition of resistance. We investigated the mechanism of 5-FU resistance using comprehensive lipidomic approaches. We performed lipidomic analysis on 5-FU–resistant (DLD-1/5-FU) and -sensitive (DLD-1) colorectal cancer cells using MALDI-MS and LC-MRM-MS. In particular, sphingomyelin (SM) species were significantly up-regulated in 5-FU–resistant cells in MALDI-TOF analysis. Further, we quantified sphingolipids including SM and Ceramide (Cer) using Multiple Reaction Monitoring (MRM), as they play a vital role in drug resistance. We found that 5-FU resistance in DLD-1/5-FU colorectal cancer cells was mainly associated with SM increase and Cer decrease, which are controlled by acid sphingomyelinase (SMPD1). In addition, reduction of SMPD1 expression was confirmed by LC-MRM-MS analysis and the effect of SMPD1 in drug resistance was assessed by treating DLD-1 cells with siRNA-SMPD1. Furthermore, clinical colorectal cancer data set analysis showed that down-regulation of SMPD1 was associated with resistance to chemotherapy regimens that include 5-FU. Thus, from our study, we propose that SM/Cer and SMPD1 are new potential target molecules for therapeutic strategies to overcome 5-FU resistance.

Structural characterization of cationic DODAB bilayers containing C24:1 β-glucosylceramide

The effect of 5 mol%, 9 mol%, and 16 mol% of C24:1 β-glucosylceramide (βGlcCer) on the structure of cationic DODAB bilayers was investigated by means of differential scanning calorimetry (DSC), electron spin resonance (ESR) spectroscopy and fluorescence microscopy. βGlcCer is completely miscible with DODAB at all fractions tested, since no domains were observed in fluorescence microscopy or ESR spectra. The latter showed that βGlcCer destabilized the gel phase of DODAB bilayers by decreasing the gel phase packing. As a consequence, βGlcCer induced a decrease in the phase transition temperature and cooperativity of DODAB bilayers, as seen in DSC thermograms. ESR spectra also showed that βGlcCer induced an increase in DODAB fluid phase order and/or rigidity. Despite their different structures, a similar effect of loosening the gel phase packing and turning the fluid phase more rigid/organized has also been observed when low molar fractions of cholesterol were incorporated in DODAB bilayers. The structural characterization of mixed membranes made of cationic lipids and glucosylceramides may be important for developing novel immunotherapeutic tools such as vaccine adjuvants.

A new insight into the reversal of multidrug resistance in cancer by nanodrugs

This work offers novel mechanistic insights into MDR reversal by nanodrugs, and this process involves reducing the P-gp distribution ratio in the cell membrane through cellular internalization behavior rather than merely evading P-gp recognition.

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.

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.

Extensive sphingolipid depletion does not affect lipid raft integrity or lipid raft localization and efflux function of the ABC transporter MRP1

We show that highly efficient depletion of sphingolipids in two different cell lines does not abrogate the ability to isolate Lubrol-based DRMs (detergent-resistant membranes) or detergent-free lipid rafts from these cells. Compared with control, DRM/detergent-free lipid raft fractions contain equal amounts of protein, cholesterol and phospholipid, whereas the classical DRM/lipid raft markers Src, caveolin-1 and flotillin display the same gradient distribution. DRMs/detergent-free lipid rafts themselves are severely depleted of sphingolipids. The fatty acid profile of the remaining sphingolipids as well as that of the glycerophospholipids shows several differences compared with control, most prominently an increase in highly saturated C(16) species. The glycerophospholipid headgroup composition is unchanged in sphingolipid-depleted cells and cell-derived detergent-free lipid rafts. Sphingolipid depletion does not alter the localization of MRP1 (multidrug-resistance-related protein 1) in DRMs/detergent-free lipid rafts or MRP1-mediated efflux of carboxyfluorescein. We conclude that extensive sphingolipid depletion does not affect lipid raft integrity in two cell lines and does not affect the function of the lipid-raft-associated protein MRP1.

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.

Thermotropic behavior and lateral distribution of very long chain sphingolipids

Sphingolipids containing very long acyl chains are abundant in certain specialized tissues and minor components of plasma membranes in most mammalian cells. There are cellular processes in which these sphingolipids are required, and the function seems to be mediated through sphingolipid-rich membrane domains. This study was conducted to explore how very long acyl chains of sphingolipids influence their lateral distribution in membranes. Differential scanning calorimetry showed that 24:0- and 24:1-sphingomyelins, galactosylceramides and glucosylceramides exhibited complex thermotropic behavior and partial miscibility with palmitoyl sphingomyelin. The T(m) was decreased by about 20 degrees C for all 24:1-sphingolipids compared to the corresponding 24:0-sphingolipids. The ability to pack tightly with ordered and extended acyl chains is a necessity for membrane lipids to partition into ordered domains in membranes and thus the 24:1-sphingolipids appeared less likely to do so. Fluorescence quenching measurements showed that the 24:0-sphingolipids formed ordered domains in multicomponent membranes, both as the only sphingolipid and mixed with palmitoyl sphingomyelin. These domains had a high packing density which appeared to hinder the partitioning of sterols into them, as reported by the fluorescent cholesterol analog cholestatrienol. 24:0-SM was, however, better able to accommodate sterol than the glycosphingolipids. The 24:1-sphingolipids could, depending on head group structure, either stabilize or disrupt ordered sphingolipid/cholesterol domains. We conclude that very long chain sphingolipids, when present in biological membranes, may affect the physical properties of or the distribution of sterols between lateral domains. It was also evident that not only the very long acyl chain but also the specific molecular structure of the sphingolipids was of importance for their membrane properties.

Cholesterol but not association with detergent resistant membranes is necessary for the transport function of MRP2/ABCC2

MRP2(/ABCC2) excretes amphiphilic organic anions into bile, and associates with detergent-resistant bile canalicular membrane domains (DRM). Here, we have evaluated sensitivities of MRP2 transport function and DRM association by titrating the cellular cholesterol content. We demonstrate that the role of cholesterol in the partitioning of MRP2 to DRM can be separated from the role of cholesterol in the function of MRP2, such that (i) cholesterol is not necessary for the polarized distribution of MRP2 at the canalicular membrane, (ii) partitioning into DRM is not required for MRP2 function, yet (iii) the presence of cholesterol is necessary for transport activity.

Cholesterol: Coupling between membrane microenvironment and ABC transporter activity

Lipid composition of biological membranes is closely related to the function of the ATP-binding cassette (ABC) transporter P-Glycoprotein (Pgp). Herein, we studied how membrane physico-chemical properties affect Pgp-activity. We effectively modulated the cellular cholesterol content using methyl-beta-cyclodextrin (MbetaCD) and MbetaCD-cholesterol-inclusion complex. Pgp was not liberated from the plasma membrane during cholesterol modulation and functional inhibition of Pgp was related to varying cholesterol levels in the plasma membrane. Our data indicate that membrane fluidity does not solely account for cholesterol dependent modifications of Pgp-activity. Therefore, we isolated lipid rafts and examined distinct membrane microdomains. Both depletion and cholesterol enrichment induces a disassembly of lipid rafts. In cholesterol-depleted cell membranes a shift in the Pgp localisation to detergent soluble fractions was observed. Enrichment of membrane cholesterol changed lipid raft distribution but not the localisation of Pgp. From our data we conclude that Pgp-transport capacity depends on accurate lipid raft properties.

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.

New insights into glycosphingolipid functions--storage, lipid rafts, and translocators

Glycosphingolipids are key components of eukaryotic cellular membranes. Through their propensity to form lipid rafts, they are important in membrane transport and signaling. At the cell surface, they are required for caveolar-mediated endocytosis, a process required for the action of many glycosphingolipid-binding toxins. Glycosphingolipids also exist intracellularly, on both leaflets of organelle membranes. It is expected that dissecting the mechanisms of cell pathology seen in the glycosphingolipid storage diseases, where lysosomal glycosphingolipid degradation is defective, will reveal their functions. Disrupted cation gradients in Mucolipidosis type IV disease are interlinked with glycosphingolipid storage, defective rab 7 function, and the activation of autophagy. Relationships between drug translocators and glycosphingolipid synthesis are also discussed. Mass spectrometry of cell lines defective in drug transporters reveal clear differences in glycosphingolipid mass and fatty acid composition. The potential roles of glycosphingolipids in lipid raft formation, endocytosis, and cationic gradients are discussed.

Sphingolipids in cancer: Regulation of pathogenesis and therapy

Sphingolipids are known to play important roles in the regulation of cell proliferation, response to chemotherapeutic agents, and/or prevention of cancer. Recently, significant progress has been made in the identification of the enzymes and their biochemical functions involved in sphingolipid metabolism. In addition, development of new techniques for the quantitative analysis of sphingolipids at their physiological levels has facilitated studies to examine distinct functions of these bioactive sphingolipids in cancer pathogenesis and therapy. This review will focus on the recent developments regarding the roles of bioactive sphingolipids in the regulation of cell growth/proliferation, and anti-cancer therapeutics.

Lipidomic strategies to study structural and functional defects of ABC-transporters in cellular lipid trafficking

The majority of the human ATP-binding cassette (ABC)-transporters function in cellular lipid trafficking and in the regulation of membrane lipid composition associating their dysfunction with human disease phenotypes related to sterol, phospholipid and fatty acid homeostasis. Based on findings from monogenetic disorders, animal models, and in vitro systems, major clues on the expression, function and cellular localization of human ABC-transporters have been gained. Here we review novel lipidomic technologies including quantitative mRNA expression monitoring by realtime RT-PCR and DNA-microarrays, lipid mass spectrometry, cellular fluorescence imaging and flow cytometry as promising tools to further define regulatory networks, lipid species patterns and subcellular domains important for ABC-transporter-mediated lipid trafficking.

Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages

PURPOSE OF REVIEW

The initial steps of reverse cholesterol transport involve export of cholesterol from peripheral cells to plasma lipoproteins for subsequent delivery to the liver. The review discusses recent developments in our understanding of how these steps occur, with particular emphasis on the macrophage, the major site of cellular cholesterol accumulation in atherosclerosis.

RECENT FINDINGS

ATP binding cassette transporter (ABC) A1 exports cholesterol and phospholipid to lipid-free apolipoproteins, while ATP binding cassette transporter G1 and scavenger receptor BI export cholesterol to phospholipid-containing acceptors. ABCA1-dependent cholesterol export involves an initial interaction of apolipoprotein AI with lipid raft membrane domains, although ABCA1 and most exported cholesterol are not raft associated. ABCG1 exports cholesterol to HDL and other phospholipid-containing acceptors. These include particles generated during lipidation of apoAI by ABCA1, suggesting that the two transporters cooperate in cholesterol export. Scavenger receptor BI is atheroprotective, mediating clearance of HDL cholesterol by the liver. The relative contributions of scavenger receptor BI and ABCG to cholesterol export to HDL from macrophages is unclear and may depend on cellular cholesterol status and the cholesterol gradient between cell and acceptor.

SUMMARY

The presence of distinct pathways for cholesterol efflux to lipid-free apolipoprotein AI and phospholipid-containing HDL species clarifies our understanding of reverse cholesterol transport, and provides new opportunities for its therapeutic manipulation.