Multidrug resistance (MDR1) P-glycoprotein enhances esterification of plasma membrane cholesterol

Role of multiple drug resistance protein 1 in neutral but not acidic glycosphingolipid biosynthesis

Transfection studies have implicated the multiple drug resistance pump, MDR1, as a glucosyl ceramide translocase within the Golgi complex (Lala, P., Ito, S., and Lingwood, C. A. (2000) J. Biol. Chem. 275, 6246-6251). We now show that MDR1 inhibitors, cyclosporin A or ketoconazole, inhibit neutral glycosphingolipid biosynthesis in 11 of 12 cell lines tested. The exception, HeLa cells, do not express MDR1. Microsomal lactosyl ceramide and globotriaosyl ceramide synthesis from endogenous or exogenously added liposomal glucosyl ceramide was inhibited by cyclosporin A, consistent with a direct role for MDR1/glucosyl ceramide translocase activity in their synthesis. In contrast, cellular ganglioside synthesis in the same cells, was unaffected by MDR1 inhibition, suggesting neutral and acid glycosphingolipids are synthesized from distinct precursor glycosphingolipid pools. Metabolic labeling in wild type and knock-out (MDR1a, 1b, MRP1) mouse fibroblasts showed the same loss of neutral glycosphingolipid (glucosyl ceramide, lactosyl ceramide) but not ganglioside (GM3) synthesis, confirming the proposed role for MDR1 translocase activity. Cryo-immunoelectron microscopy showed MDR1 was predominantly intracellular, largely in rab6-containing Golgi vesicles and Golgi cisternae, the site of glycosphingolipid synthesis. These studies identify MDR1 as the major glucosyl ceramide flippase required for neutral glycosphingolipid anabolism and demonstrate a previously unappreciated dichotomy between neutral and acid glycosphingolipid synthesis.

Transmembrane lipid transfer is crucial for providing neutral lipids during very low density lipoprotein assembly in endoplasmic reticulum

Very low density lipoprotein (VLDL), a large particle containing apolipoprotein B (apoB) and large amounts of neutral lipids, is formed in the luminal space within the endoplasmic reticulum (ER) of hepatic cells. The assembly mechanism of VLDL particles is a tightly regulated process where apoB, associated with an insufficient amount of lipids, is selectively degraded intracellularly. In this study we found that treatment of HuH-7 human hepatoma cells with verapamil inhibited secretion of apoB-containing lipoprotein particles through increasing degradation of apoB. Addition of N-acetylleucyl-leucyl-norleucinal, an inhibitor of proteasome and other cysteinyl proteases that are responsible for apoB degradation, restored apoB recovery from verapamil-treated cells. De novo synthesis of lipids from [14C]acetate was increased in the presence of verapamil, suggesting that verapamil decreases lipid availability for apoB thus leading to the secretion of apoB-containing lipoprotein. We prepared cytosolic fractions from cells preincubated with [14C]acetate and used as a donor of radioactive lipids. When this cytosolic fraction was incubated with microsomes isolated separately, radioactive triglyceride (TG) accumulated in the luminal space of the microsomes. The transfer of radioactive TG from the cytosolic fraction to the microsomal lumen was inhibited in the presence of verapamil, suggesting that there is a verapamil-sensitive mechanism for TG transfer across ER membranes that is involved in formation of apoB-containing lipoprotein particles in ER. Verapamil showed no inhibitory effect on microsomal TG transfer protein, a well known lipid transfer protein in ER. We propose from these results that there is novel machinery for transmembrane movement of neutral lipids, which is involved in providing TG for apoB during VLDL assembly in ER.

Role of cholesterol ester pathway in the control of cell cycle in human aortic smooth muscle cells

Cholesterol esterification by acyl-CoA:cholesterol acyltransferase (ACAT) and proliferation of vascular smooth muscle cells (VSMC) are key events in vascular proliferative diseases. Here we performed experiments to ascertain the role of cholesterol ester pathway in the control of human aortic VSMC cycle progression. Results showed that serum-induced VSMC proliferation was preceded by an increased ability of the cells to esterify cholesterol as well as by an increased expression of ACAT and multidrug resistance (MDR1) mRNAs and extracellular related kinases 1/2 (ERK1/2), whereas caveolin-1 levels were markedly decreased. Cell cycle analyses performed in the presence of two inhibitors of cholesterol esterification, directly inhibiting ACAT (Sandoz 58-035) or the transport of cholesterol substrate from plasma membrane to endoplasmic reticulum (progesterone), indicate that each inhibitor suppressed the serum-induced DNA synthesis by accumulation of VSMCs in the G1 phase. The effect was associated with a rapid inhibition of ERK1/2 mitogenic signaling pathway; a down-regulation of cyclin D1, ACAT, and MDR1 mRNA; and an up-regulation of caveolin-1. These data provide a plausible link between cholesterol esterification and control of cell cycle G1/S transition, supporting the hypothesis that cholesterol esterification may accelerate the progression of human vascular proliferative diseases by modulating the rate of the VSMC proliferation.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

Functional analysis of candidate ABC transporter proteins for sitosterol transport

Two ATP-binding cassette (ABC) proteins, ABCG5 and ABCG8, have recently been associated with the accumulation of dietary cholesterol in the sterol storage disease sitosterolemia. These two 'half-transporters' are assumed to dimerize to form the complete sitosterol transporter which reduces the absorption of sitosterol and related molecules in the intestine by pumping them back into the lumen. Although mutations altering ABCG5 and ABCG8 are found in affected patients, no functional demonstration of sitosterol transport has been achieved. In this study, we investigated whether other ABC transporters implicated in lipid movement and expressed in tissues with a role in sterol synthesis and absorption, might also be involved in sitosterol transport. Transport by the multidrug resistance P-glycoprotein (P-gp; Abcb1), the multidrug resistance-associated protein (Mrp1; Abcc1), the breast cancer resistance protein (Bcrp; Abcg2) and the bile salt export pump (Bsep; Abcb11) was assessed using several assays. Unexpectedly, none of the candidate proteins mediated significant sitosterol transport. This has implications for the pathology of sitosterolemia. In addition, the data suggest that otherwise broad-specific ABC transporters have acquired specificity to exclude sitosterol and related sterols like cholesterol presumably because the abundance of cholesterol in the membrane would interfere with their action; in consequence, specific transporters have evolved to handle these sterols.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

P-glycoprotein inhibits caspase-8 activation but not formation of the death inducing signal complex (disc) following Fas ligation

Previous studies by our laboratory have shown that the drug transporter protein P-glycoprotein, P-gp, can specifically inhibit Fas-induced caspase-3 activation and apoptosis. Importantly, inhibition of both caspase-3 activation and cell death could be reversed by pharmacological and antibody inhibitors of P-gp function. However, the molecular mechanisms underpinning P-gp-mediated resistance to Fas-induced cell death and caspase activation remained unknown. We therefore sought to identify the point(s) within the death receptor pathway at which P-gp exerted its inhibitory effect and to determine whether the ATPase activity of P-gp was required. Structure-function analysis determined that ATP hydrolysis was necessary for P-gp to confer resistance to Fas-induced caspase activation and cell death. Importantly, although both FADD and caspase-8 were recruited to the Death Inducing Signal Complex (DISC) in wild-type P-gp expressing cells following Fas ligation, subsequent activation of caspase-8 at the DISC was inhibited. The ability of P-gp to inhibit caspase-8 activation was also ATP dependent. These studies demonstrate that P-gp inhibits Fas-induced caspase-8 activation but not formation of the DISC and that this activity of P-gp is dependent on ATP hydrolysis.

MDR1 genotype-related pharmacokinetics and pharmacodynamics

The multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. MDR1 acts as an energy-dependent efflux pump that exports its substrates out of cells. MDR1 was originally isolated from resistant tumor cells as part of the mechanism of multidrug resistance, but over the last decade, it has been elucidated that human MDR1 is also expressed throughout the body to confer intrinsic resistance to the tissues by exporting unnecessary or toxic exogeneous substances or metabolites. A number of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics and pharmacodynamics. In 2000, Hoffmeyer et al. performed a systemic screening for MDR1 polymorphisms and detected 15 single nucleotide polymorphisms (SNPs). They also indicated that a polymorphism in exon 26 at position 3435 (C3435T), a silent mutation, affected the expression level of MDR1 protein in duodenum, and thereby the intestinal absorption of digoxin. To date, the genotype frequencies of C3435T have been investigated extensively using a larger population and interethnic difference has been elucidated, and a total of 28 SNPs have been found at 27 positions on the MDR1 gene. Clinical studies on MDR1 genotype-related MDR1 expression and pharmacokinetics have also been performed around the world; however, results were not always consistent with Hoffmeyer's report. In this review, published reports are summarized for the future individualization of pharmacotherapy based on MDR1 genotyping. In addition, recent investigations have raised the possibility that MDR1 and related transporters play a fundamental role in regulating apoptosis and immunology, and in fact, there are reports of MDR1-related susceptibility to inflammatory bowel disease, HIV infection and renal cell carcinoma. Herein, these issues are also summarized, and the current status of the knowledge in the area of pharmacogenomics of other transporters is briefly introduced.

Lipids in blood-brain barrier models in vitro II: Influence of glial cells on lipid classes and lipid fatty acids

Lipids of brain tissue and brain microvascular endothelial cells contain high proportions of long-chain polyunsaturated fatty acids (long PUFAs). The blood-brain barrier (BBB) is formed by the brain endothelial cells under the inductive influence of brain cells, especially perivascular glia, and coculture of endothelial cells and glial cells has been used to examine this induction. The objective of this study was to investigate whether C6 glioma cells are able to influence the lipid composition and shift the fatty acid (FA) patterns of the BBB model cell lines RBE4 and ECV304 toward the in vivo situation. Lipid classes of the three cell lines were analyzed by thin-layer chromatography and lipid FA patterns by high-performance liquid chromatography. Only ECV304 cells showed altered lipid composition in coculture with C6 cells. The fractions of triglycerides and cholesteryl esters (depending on the support filter) were about twice as high in coculture as when the cells were grown alone. Triglyceride fractions reached 13 to 15% of total lipids in coculture. The three cell lines showed an increase in the percentage of long PUFAs with respect to unsaturated FAs, mainly because of an increase in the percentages of arachidonic acid, all cis-7,10,13,16-docosatetraenoic acid, and all cis-7,10,13,16,19-docosapentaenoic acid. It is concluded that glioma C6 cells are able to induce a more in vivo-like FA pattern in BBB cell culture models. However, changes were not significant for the individual PUFAs, and their levels did not reach in vivo values.

Physical state changes of membrane lipids in human lung adenocarcinoma A(549) cells and their resistance to cisplatin

The properties of membrane lipids in sensitive A(549) and resistant A(549)/DDP cells to cis-dichlorodiammine platinum[II] (cisplatin) were examined by combining different approaches. The results showed that fluorescence intensity (deltaF) of Merocyanine 540 (MC540) was 93.5 +/- 21.8 for the sensitive A(549) cells and 49.5 +/- 11.2 for the resistive A(549)/DDP cells, monitored by flow cytometry, which may indicate that membrane lipid packing of the sensitive A(549) cells were looser than that of the resistant A(549)/DDP cells. Diffusion rate of N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-1,2-hexadecanoyl-Sn-glycero-3-phosphatidyl-ethanolamine (NBD-PE) was slower in A(549)/DDP cells than in A(549) cells as detected by fluorescence recovery after photobleaching (FRAP) technique. Fatty acid analysis of the membrane lipids showed 21.6, 27.0 and 31.8% increase in the amount of C(18:1), C(18:2) and C(18:3) fatty acid, respectively, in A(549) cells as compared to A(549)/DDP cells. The total amount of unsaturated fatty acids in the plasma membrane lipid is 69.13% +/- 2.2% for A(549), and 55.08% +/- 1.8% for A(549)/DDP cells, respectively. The resistance to cisplatin in A(549)/DDP cells was confirmed by the measurements of the transmembrane influx of Rhodamine-123, cisplatin or Bodipy-cisplatin by fluorescence assay and inductively coupled plasma mass spectrometry (ICP-MS). From the results described previously, it is concluded that changes in the membrane lipids "composition" cause a change in the physical state of the plasma membrane lipids and that this may be associated with the resistance of A(549)/DDP cells to cisplatin.

Sphingolipids in neuroblastoma: their role in drug resistance mechanisms

Disseminated neuroblastoma usually calls for chemotherapy as the primary approach for treatment. Treatment failure is often attributable to drug resistance. This involves a variety of cellular mechanisms, including increased drug efflux through expression of ATP-binding cassette transporters (e.g., P-glycoprotein) and the inability of tumor cells to activate or propagate the apoptotic response. In recent years it has become apparent that sphingolipid metabolism and the generation of sphingolipid species, such as ceramide, also play a role in drug resistance. This may involve an autonomous mechanism, related to direct effects of sphingolipids on the apoptotic response, but also a subtle interplay between sphingolipids and ATP-binding cassette transporters. Here, we present an overview of the current understanding of the multiple levels at which sphingolipids function in drug resistance, with an emphasis on sphingolipid function in neuroblastoma and how modulation of sphingolipid metabolism may be used as a novel treatment paradigm.

ATP-binding cassette (ABC) transporters in atherosclerosis

Macrophages play a central role in the initiation and progression of atherosclerotic lesions. In the nascent lesion, macrophages transform into foam cells through the excessive accumulation of cholesteryl esters. Dysfunctional lipid homeostasis in macrophages and foam cells ultimately results in the breakdown of membrane integrity and cell death. Studies within the past 2 years have implicated a defined subset of multispan transmembrane proteins, the ATP-binding cassette (ABC) transporters, in macrophage lipid homeostasis. The recent finding that ABCA1, beyond its function as a major regulator of plasma high-density lipoprotein metabolism, exerts significant antiatherosclerotic activities has provided the first direct evidence for the role of an ABC transporter in the pathogenesis of atherosclerosis.

Evidence of P-glycoprotein mediated apical to basolateral transport of flunisolide in human broncho-tracheal epithelial cells (Calu-3)

1. Transepithelial transport of flunisolide was studied in reconstituted cell monolayers of Calu-3, LLC-PK1 and the MDR1-P-glycoprotein transfected LLC-MDR1 cells. 2. Flunisolide transport was polarized in the apical (ap) to basolateral (bl) direction in Calu-3 cells and was demonstrated to be ATP-dependent. In LLC-MDR1 cells, flunisolide was transported in the bl to ap direction and showed no polarization in LLC-PK1 cells. 3. Non-specific inhibition of cellular metabolism at low temperature (4 degrees C) or by 2-deoxy-D-glucose (2-d-glu) and sodium azide (NaN(3)) abolished the polarized transport. Polarized flunisolide transport was also inhibited by the specific Pgp inhibitors verapamil, SDZ PSC 833 and LY335979. 4. Under all experimental conditions and in the presence of all used inhibitors, no decrease in the TransEpithelial Electrical Resistance (TEER) values was detected. From all inhibitors used, only the general metabolism inhibitors 2-deoxy-D-glucose and NaN(3), decreased the survival of Calu-3 cells. 5. Western blotting analysis and confocal laser scanning microscopy demonstrated the presence of MDR1-Pgp at mainly the basolateral side of the plasma membrane in Calu-3 cells and at the apical side in LLC-MDR1 cells. Mass spectroscopy studies demonstrated that flunisolide is transported unmetabolized across Calu-3 cells. 6. In conclusion, these results show that the active ap to bl transport of flunisolide across Calu-3 cells is facilitated by MDR1-Pgp located in the basolateral plasma membrane.

P glycoprotein-mediated cholesterol cycling determines proximal tubular cell viability

BACKGROUND

MDR P glycoproteins may help transport plasma membrane free cholesterol (FC) to the endoplasmic reticulum (ER), where it undergoes acylation, forming cholesterol esters (CE). This study assessed whether P glycoprotein inhibitors alter renal tubular FC/CE expression, thereby altering cell integrity.

METHODS

Mouse proximal tubule segments (PTS) were exposed to chemically dissimilar P glycoprotein inhibitors [progesterone (prog), trifluoperazine (TFP), or cyclosporine A (CsA)]. Their effects on FC/CE and adenosine 5'-triphosphate (ATP) levels, phospholipid expression, lipid peroxidation, and cell viability (lactate dehydrogenase release; LDH) were assessed. P glycoprotein inhibitor effects on cultured proximal tubular (HK-2) cell viability and susceptibility to Fe-induced oxidant stress were also addressed.

RESULTS

When applied to PTS, prog, TFP, and, to lesser extent, CsA induced dose-dependent ATP reductions (< or =90%), CE decrements (approximately 40%), and LDH release (< or =60%). No concomitant changes in lipid peroxidation or phospholipid profiles were observed. Ouabain did not preserve tubular ATP, suggesting that decreased ATP production, rather than increased consumption, was operative. Mechanisms leading to cell lysis were not identical, as glycine and arachidonic acid blocked prog- but not TFP-mediated cell death. When prog-driven CE reductions were attenuated in PTS with a procycling agent (cholesterol oxidase), decreased cell death resulted. P glycoprotein inhibitors also caused dose-dependent HK-2 cell death. Blocking Fe-mediated CE formation ( approximately x10) with sublethal CsA doses led to a marked increase in Fe-mediated cell death.

CONCLUSIONS

P glycoproteins may be critical to tubule cholesterol transport. If blocked with pharmacologic agents, decreased ATP production, overt cell lysis, and/or a marked propensity to superimposed tubular cell injury can result.

P-glycoprotein does not protect cells against cytolysis induced by pore-forming proteins

P-glycoprotein (P-gp) is an ATP-dependent drug pump that confers multidrug resistance (MDR). In addition to its ability to efflux toxins, P-gp can also inhibit apoptosis induced by a wide array of cell death stimuli that rely on activation of intracellular caspases for full function. We therefore hypothesized that P-gp may have additional functions in addition to its role in effluxing xenotoxins that could provide protection to tumor cells against a host response. There have been a number of contradictory reports concerning the role of P-gp in regulating complement activation. Given the disparate results obtained by different laboratories and our published results demonstrating that P-gp does not affect cell death induced by another membranolytic protein, perforin, we decided to assess the role of P-gp in regulating cell lysis induced by a number of different pore-forming proteins. Testing a variety of different P-gp-expressing MDR cell lines produced following exposure of cells to chemotherapeutic agents or by retroviral gene transduction in the complete absence of any drug selection, we found no difference in sensitivity of P-gp(+ve) or P-gp(-ve) cells to the pore-forming proteins complement, perforin, or pneumolysin. Based on these results, we conclude that P-gp does not affect cell lysis induced by pore-forming proteins.

Cholesterol ester accumulation: an immediate consequence of acute in vivo ischemic renal injury

BACKGROUND

Cholesterol is a major constituent of plasma membranes, and recent evidence indicates that it is up-regulated during the maintenance phase of acute renal failure (ARF). However, cholesterol's fate and that of the cholesterol ester (CE) cycle [shuttling between free cholesterol (FC) and CEs] during the induction phase of ARF have not been well defined. The present studies sought to provide initial insights into these issues.

METHODS

FC and CE were measured in mouse renal cortex after in vivo ischemia (15 and 45 minutes)/reperfusion (0 to 120 minutes) and glycerol-induced myoglobinuria (1 to 2 hours). FC/CE were also measured in (1) cultured human proximal tubule (HK-2) cells three hours after ATP depletion and in (2) isolated mouse proximal tubule segments (PTSs) subjected to plasma membrane damage (with cholesterol oxidase, sphingomyelinase, phospholipase A2, or cytoskeletal disruption with cytochalasin B). The impact of cholesterol synthesis inhibition (with mevastatin) and FC traffic blockade (with progesterone) on injury-evoked FC/CE changes was also assessed.

RESULTS

In vivo ischemia caused approximately threefold to fourfold CE elevations, but not FC elevations, that persisted for at least two hours of reperfusion. Conversely, myoglobinuria had no effect. Isolated CE increments were observed in ATP-depleted HK-2 cells. Neither mevastatin nor progesterone blocked this CE accumulation. Plasma membrane injury induced with sphingomyelinase or cholesterol oxidase, but not with phospholipase A(2) or cytochalasin B, increased tubule CE content. High CE levels, induced with cholesterol oxidase, partially blocked hypoxic PTS attack.

CONCLUSIONS

In vivo ischemia/reperfusion acutely increases renal cortical CE, but not FC, content, indicating perturbed CE/FC cycling. The available data suggest that this could stem from specific types of plasma membrane damage, which then increase FC flux via aberrant pathways to the endoplasmic reticulum, where CE formation occurs. That CE levels are known to inversely correlate with both renal and nonrenal cell injury suggests the potential relevance of these observations to the induction phase of ischemic ARF.

ATP-binding cassette transporter A1 (ABCA1) affects total body sterol metabolism

BACKGROUND AND AIMS

Members of the family of ABC transporters are involved in different processes of sterol metabolism, and ABCA1 was recently identified as a key regulator of high-density lipoprotein (HDL) metabolism. Our aim was to further analyze the role of ABCA1 in cholesterol metabolism.

METHODS

ABCA1-deficient mice (ABCA1-/-) and wild-type mice were compared for different aspects of sterol metabolism. Intestinal cholesterol absorption was determined by a dual stable isotope technique, and analysis of fecal, plasma, and tissue sterols was performed by gas chromatography/mass spectrometry. Key regulators of sterol metabolism were investigated by Northern and Western blot analyses or enzyme activity assays.

RESULTS

ABCA1-disrupted sv129/C57BL/6 hybrid mice showed a significant reduction in intestinal cholesterol absorption. The decrease in cholesterol absorption was followed by an enhanced fecal loss of neutral sterols, whereas fecal bile acid excretion was not affected. Total body cholesterol synthesis was significantly increased, with enhanced 3-hydroxy-3-methyglutaryl-coenzyme A (HMG-CoA) reductase observed in adrenals and spleen. In addition, ABCA1-/- mice showed markedly increased concentrations of cholesterol precursors in the plasma, lung, intestine, and feces. Reduced HMG-CoA reductase messenger RNA and enzyme activity in the liver suggest that enhanced cholesterol synthesis in ABCA1-/- mice occurs in peripheral tissues rather than the liver.

CONCLUSIONS

The metabolism of cholesterol and cholesterol precursors is markedly affected by a lack of ABCA1 function.

The relationship between 1H-NMR mobile lipid intensity and cholesterol in two human tumor multidrug resistant cell lines (MCF-7 and LoVo)

The high resolution proton nuclear magnetic resonance (1H-NMR) spectra of two different cell lines exhibiting multidrug resistance (MDR) as demonstrated by the expression of the well-known energy-driven, membrane-bound 170 kDa P-glycoprotein pump known as Pgp were investigated. In particular, the mobile lipid (ML) profile, and the growth and biochemical characteristics of MCF-7 (human mammary carcinoma) and LoVo (human colon adenocarcinoma) sensitive and resistant tumor cells were compared. The results indicate that both MCF-7 and LoVo resistant cells have a higher ML intensity than their respective sensitive counterparts. However, since sensitive and resistant cells of each pair grow in the same manner, variations in growth characteristics do not appear to be the cause of the ML changes as has been suggested by other authors in non-resistant tumor cells. In order to investigate further the origin of the ML changes, lipid analyses were conducted in sensitive and resistant cell types. The results of these experiments show that resistant cells of both cell types have a greater amount of esterified cholesterol and saturated cholesteryl ester and triglyceride fatty acid than their sensitive counterparts. From a thorough analysis of the data obtained in this paper utilizing numerous techniques including biological, biophysical and biochemical ones, it is hypothesized that cholesterol and triglyceride play a pivotal role in inducing changes in NMR ML signals. The importance of these lipid variations in MDR is discussed in view of the controversy regarding the origin of ML signals and the paramount role played by the Pgp pump in resistance.

Multidrug transporters in prokaryotic and eukaryotic cells: physiological functions and transport mechanisms

Multidrug transporters mediate the extrusion of structurally unrelated drugs from prokaryotic and eukaryotic cells. As a result of this efflux activity, the cytoplasmic drug concentration in the cell is lowered to subtoxic levels and, hence, cells become multidrug resistant. The activity of multidrug transporters interferes with the drug-based control of tumours and infectious pathogenic microorganisms. There is an urgent need to understand the structure-function relationships in multidrug transporters that underlie their drug specificity and transport mechanism. Knowledge about the architecture of drug and modulator binding sites and the link between energy-generating and drug translocating functions of multidrug transporters may allow one to rationally design new drugs that can poison or circumvent the activity of these transport proteins. Furthermore, if one is to inhibit multidrug transporters in human cells, one should know more about their physiological substrates and functions. This review will summarize important new insights into the role that multidrug transporters in general, and P-glycoprotein and its bacterial homologue LmrA in particular, play in the physiology of the cell. In addition, the molecular basis of drug transport by these proteins will be discussed.

Multidrug resistance P-glycoprotein is not involved in cholesterol esterification

The aim of the present paper is to reinvestigate the role of multidrug resistance P-glycoprotein MDR1 and MDR-associated protein (MRP1) in cholesterol esterification using well-characterized inhibitors. Using specific substrate efflux assay, we show that GF120918 (0.2 microM) and probenecid (5 mM) were specific inhibitors of MDR1 and MRP1, respectively. In HepG2 cells, neither of them affect the esterification of cholesterol derived from the uptake of cholesterol-rich lipoprotein, while both verapamil (100 microM) and progesterone (100 microM) were able to inhibit cholesterol esterification. Similar results were obtained with verapamil, progesterone, and GF120918 in the MDR1-overexpressing cells MCF7/ADR. The capacity of progesterone to reduce cholesterol esterification is not correlated with its ability to inhibit MDR1 but is rather due to direct inhibition of acyl-CoA:cholesterol acyltransferase (ACAT). We conclude that the esterification of cholesterol is not correlated with MDR1 or MRP1 activity, thus excluding their role in the intracellular transport of endocytosis-derived cholesterol.

Cholesterol and caveolae: structural and functional relationships

Caveolae are free cholesterol (FC)- and sphingolipid-rich surface microdomains abundant in most peripheral cells. Caveolin, a FC binding protein, is a major structural element of these domains. Caveolae serve as portals to regulate cellular FC homeostasis, possibly via their association with ancillary proteins including scavenger receptor B1. The FC content of caveolae regulates the transmission of both extracellular receptor-mediated and endogenous signal transduction via changes in the composition of caveolin-associated complexes of signaling intermediates. By controlling surface FC content, reporting membrane changes by signal transduction to the nucleus, and regulating signal traffic in response to extracellular stimuli, caveolae exert a multifaceted influence on cell physiology including growth and cell division, adhesion, and hormonal response. Cell surface lipid 'rafts' may assume many of the functions of caveolae in cells with low levels of caveolin.

Does P-glycoprotein play a role in anticancer drug pharmacokinetics?

The multidrug-resistance P-glycoprotein is a drug efflux transport protein abundantly present in various types of human cancer. The protein is encoded by the MDR1 gene and its function is sensitive to modulation by competitive inhibition. Clinical studies have indicated that inhibitors of P-glycoprotein function dramatically decrease the systemic clearance of anticancer agents, necessitating dose reduction. This dose reduction not only complicated the interpretation of toxicity and response data, but also presented a serious obstacle in the development and rational use of P-glycoprotein inhibitors. It is now evident that the pharmacokinetic interference between anticancer drugs and P-glycoprotein inhibitors is due primarily to competition for drug metabolizing enzymes. A wealth of recent experimental data shows that many of the previously tested P-glycoprotein inhibitors, including verapamil, cyclosporin A, and valspodar (SDZ PSC 833), are substrates and/or potent inhibitors of cytochrome P450 3A4 (CYP3A4). Future development and clinical use of potent P-glycoprotein modulators lacking high affinity for CYP3A4 should decrease the impact of these important drug interactions and will eventually result in improved therapeutic specificity and efficacy. Copyright 2000 Harcourt Publishers Ltd.

A two-step mechanism for free cholesterol and phospholipid efflux from human vascular cells to apolipoprotein A-1

Smooth muscle and endothelial cells in vivo are quiescent yet exposed to high levels of lipoprotein lipids. Phospholipid (PL) and free cholesterol (FC) efflux maintain homeostasis. Smooth muscle cells (SMC) expressed high levels of ABC-1 transporter mRNA, and glyburide-dependent PL and FC efflux to apolipoprotein A-1 (apo A-1), the major protein of high-density lipoprotein. FC efflux was inhibited by vanadate and okadaic acid, while PL efflux was not. Phosphatidylcholine was the major PL transferred by both cell types. Stimulation of phosphatidylserine efflux, redistributed within the membrane by this transporter, was only minimally increased. Umbilical vein and aortic endothelial cells expressed little ABC-1 mRNA, nor did these cells promote either PL or FC efflux in response to the presence of apo A-1. To investigate the mechanism of ABC-1-dependent lipid efflux from these cells, apo A-1 was preincubated in the presence of unlabeled SMC or fibroblasts, and the conditioned medium was then transferred to endothelial cells. This medium catalyzed the efflux of FC but not of PL from endothelial cells. Such FC efflux was resistant to glyburide but inhibited by okadaic acid and vanadate. The data suggest that ABC-1-dependent PL efflux precedes FC efflux to apo A-1 and that the complex of apo A-1 and PL is a much better acceptor of FC than apo A-1 itself. Inhibition of FC but not PL efflux by vanadate and okadaic acid suggests these transfers involve different mechanisms.

Multidrug resistance: a role for cholesterol efflux pathways?

Multidrug resistance (MDR) severely impairs the efficacy of cancer chemotherapy. Several protein transporters that mediate drug export have been identified, but additional adaptations appear to be necessary for full-fledged drug resistance. The cell surface density of caveolae and the expression of the caveolar coat protein caveolin are dramatically increased in MDR cancer cells. Acquisition of MDR might thus be accompanied by upregulation of caveolin-dependent cholesterol efflux pathways, raising the possibility that these same pathways are utilized for delivering drugs from intracellular compartments to the plasma membrane, where drugs can be extruded from the cells by drug efflux ATPases. The upregulation of caveolin mandates a phenotypic change of MDR cells in terms of their cholesterol homeostasis and is accompanied by loss of important features of the transformed phenotype of MDR cancer cells.

Physiologic concentrations of bile salts inhibit rat hepatic alkaline phosphatase but not the intestinal isoenzyme

OBJECTIVE

The effect of bile salts on alkaline phosphatase (EC 3.1.3.1) activity from Wistar rat liver, duodenum, jejunum, and serum was investigated.

DESIGN AND RESULTS

For concentrations higher than 1 mM conjugated bile salts (glycocholate, glycochenodeoxycholate, taurocholate, taurodeoxycholate, and taurochenodeoxycholate) inhibited hepatic ALP but, up to concentrations of 10 mM, had no effect on intestinal ALP. Also cholate, deoxycholate, and chenodeoxycholate, within the same concentration range, did not have any effect on intestinal ALP. ALP inhibition induced by conjugated bile salts was significantly higher in serum of starved rats than in serum of fed animals, what is in good agreement with the known higher proportion of hepatic ALP and lower proportion of intestinal ALP in serum of starved rats.

CONCLUSIONS

Bile salts can, thus, be used to help discriminating between tissue-nonspecific and intestinal ALP isoenzymes and identifying pathologic conditions where the relative quantities of these isoenzymes are altered in serum. Inhibition of hepatic ALP by physiologic concentrations of bile salts may bear some relation to the bile salts effects on their own enterohepatic circulation and/or biosynthesis.

Cholesterol interaction with the daunorubicin binding site of P-glycoprotein

The inherent complexities of cholesterol disposition and metabolism preclude a single transmembrane active transport avenue for this steroid-precursor, cell-membrane constituent. Yet the ABC (ATP binding cassette) transporters are inextricably linked to elements of cholesterol disposition. Recent observations have suggested that, under certain settings, the ABC transporter P-glycoprotein (P-gp) performs a direct role in cholesterol disposition. The gene product of MDR1 (multidrug resistance transporter), P-glycoprotein also confers protection against xenobiotics. Using a whole cell assay in which the retention of a marker substrate is evaluated and quantified, we studied the ability of cholesterol to inhibit directly the function of this transporter. In a NIH-G185 cell line presenting an overexpressed amount of the human transporter P-gp, cholesterol caused dramatic inhibition of daunorubicin transport with an IC(50) of about 8 microM yet had no effect on the parent cell line nor rhodamine 123 transport. Additionally, using the ATP-hydrolysis assay, we showed that cholesterol increases P-gp-mediated ATP hydrolysis by approximately 1.6-fold with a K(s) of 5 microM. Suggesting that cholesterol directly interacts with the substrate binding site of P-gp, these results are consistent with cholesterol being transported by MDR1 P-gp.

Effects of MDR1 and MDR3 P-glycoproteins, MRP1, and BCRP/MXR/ABCP on the transport of (99m)Tc-tetrofosmin

Multidrug resistance (MDR1) P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP1), and breast cancer resistance protein (BCRP/MXR/ABCP) are members of the ATP-binding-cassette (ABC) superfamily of membrane transporters and are thought to function as energy-dependent efflux pumps of a variety of structurally diverse chemotherapeutic agents. We herein report the characterization of (99m)Tc-Tetrofosmin, a candidate radiopharmaceutical substrate of ABC transporters. (99m)Tc-Tetrofosmin showed high membrane potential-dependent accumulation in drug-sensitive KB 3-1 cells and low antagonist-reversible accumulation in MDR KB 8-5 and KB 8-5-11 cells in proportion to levels of MDR1 Pgp expression. In KB 8-5 cells, EC(50) values of the potent MDR antagonists N-(4-[2-(1,2,3, 4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9, 10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), (2R)-anti-5-¿3-[4-(10, 11-difluoromethanodibenzo-suber-5-yl)piperazin-1-yl]-2 -hydroxypropoxy ¿quinoline trihydrochloride (LY335979), and (3'-keto-Bmt')-[Val(2)]-cyclosporin A (PSC 833) were 40, 66, and 986 nM, respectively. Furthermore, only baculoviruses carrying human MDR1, but not MDR3, conferred both a decrease in accumulation of (99m)Tc-Tetrofosmin in host Spodoptera frugiperda (Sf9) cells and a GF120918-induced enhancement. Transport studies with a variety of stably transfected and drug-selected tumor cell lines were performed with (99m)Tc-Tetrofosmin and compared with (99m)Tc-Sestamibi, a previously validated MDR imaging agent. MDR1 Pgp readily transported each agent. To a lesser extent, MRP1 also transported each agent, likely as co-transport substrates with GSH; neither agent was a substrate for the BCRP/MXR/ABCP half-transporter. In mdr1a(-/-) and mdr1a/1b(-/-) mice, (99m)Tc-Tetrofosmin showed approximately 3. 5-fold greater brain uptake and retention compared with wild-type, with no net change in blood pharmacokinetics, consistent with transport in vivo by Pgp expressed at the capillary blood-brain barrier. Molecular imaging of the functional transport activity of ABC transporters in vivo with (99m)Tc-Tetrofosmin and related radiopharmaceuticals may enable non-invasive monitoring of chemotherapeutic and MDR gene therapy protocols.

Lipid trafficking and sorting: how cholesterol is filling gaps

Recent research has highlighted a role for cholesterol homeostasis in the regulation of trafficking and sorting of sphingolipids. This sorting may dictate the nature of the acyl chain species of phospholipids in the plasma membrane which, in turn, may govern the selective partitioning of these lipids into lateral domains. Recently, several proteins have been identified that play a role in the flow and sorting of all major lipid classes.

Effects of cholesterol and enantiomeric cholesterol on P-glycoprotein localization and function in low-density membrane domains

Multidrug resistance P-glycoprotein (Pgp) has been reported to localize in low-density, cholesterol-enriched membranes. However, effects of low-density membrane domains on function of Pgp remain unexplored in whole cell systems. In cells that express modest levels of the protein endogenously or through drug selection, Pgp predominantly localized to low-density membranes following separation on a sucrose gradient. When highly overexpressed in NIH 3T3 cells, a prominent amount of Pgp also was detected in high-density membranes. Removing cholesterol from cells with beta-methylcyclodextrin (CD), a sterol acceptor molecule, shifted fractions that contained Pgp from low toward high density, and this effect was reversed to a similar extent by restoring sterols with either cholesterol or enantiomeric cholesterol. However, function of human MDR1 Pgp as probed with Tc-Sestamibi, a transport substrate for Pgp, was not dependent on localization of Pgp in cholesterol-enriched membranes. Specific inhibition of MDR1 Pgp with GF120918 or LY335979 also was independent of cholesterol. Cell-type-specific effects of cholesterol content on function of human Pgp were detected by use of daunomycin, another substrate for Pgp, although efficacy of inhibitors remained independent of cholesterol. Conversely, both function and inhibition of hamster Pgp as measured with Tc-Sestamibi and daunomycin were in part dependent on normal cell content of cholesterol. These data show that Pgp preferentially localizes to low-density, cholesterol-enriched membrane domains, but acute depletion of cholesterol impacts Pgp-mediated drug transport in a substrate- and cell-type-specific manner.

Lipid transporters: membrane transport systems for cholesterol and fatty acids

Lipophilic molecules can passively diffuse across cell membranes, a process that is driven by the concentration gradient, by availability of acceptors to facilitate desorption from the bilayer, and by cellular metabolism. However, evidence has accumulated that supports the existence of specialized, protein-facilitated membrane transport systems for many lipophilic molecules. This has generated considerable debate regarding why such systems need to exist. The present review summarizes recent developments related to the membrane transport systems for cholesterol and fatty acids, which have been shown to involve structurally related proteins. General similarities of the cholesterol and fatty acid systems to other lipid transport systems (briefly discussed in the Introduction section) are highlighted in the Conclusion section. The overall aim of the present review is to illustrate why lipid transporters are needed in vivo, and how they accomplish specific functions that can not be met by lipid diffusion alone.

Transcriptional homeostatic control of membrane lipid composition

Plasma membranes have a structural property, commonly referred to as membrane fluidity, that is compositionally regulated. The two main features of plasma membrane lipid composition that determine membrane fluidity are the ratio of cholesterol to phospholipids and the ratio of saturated to unsaturated fatty acids that are incorporated into the phospholipids. These ratios are determined, at least in part, by regulation of membrane lipid biosynthesis-particularly that of cholesterol and oleate. It now appears that cholesterol and oleate biosynthesis are feedback regulated by a common transcriptional mechanism which is governed by the maturation of the SREBP transcription factors. In this article, we briefly review our current understanding of transcriptional regulation of plasma membrane lipid biosynthesis by sterols and oleate. We also discuss studies related to the mechanism by which the physical state of membrane lipids signals the transcriptional regulatory machinery to control the rates of synthesis of these structural components of the lipid bilayer.

A role for P-glycoprotein in regulating cell death

P-glycoprotein (P-gp) is an energy dependent drug pump responsible for multidrug resistance (MDR) in human cancers. While it is irrefutable that P-gp can efflux xenobiotics out of cells, the biological function of P-gp in multicellular organisms has yet to be firmly established. The question of what, if anything, P-gp does when not effluxing drugs has been raised by recent reports indicating that P-gp may regulate apoptosis, chloride channel activity, cholesterol metabolism and immune cell function. There is now a lively debate regarding the possible role of P-gp in regulating cell differentiation, proliferation and survival.

Human white/murine ABC8 mRNA levels are highly induced in lipid-loaded macrophages. A transcriptional role for specific oxysterols

To identify genes that are transcriptionally activated when human macrophages accumulate excess lipids, we employed the mRNA differential display technique using RNA isolated from human monocyte-macrophages incubated in the absence or presence of acetylated low density lipoprotein and sterols (cholesterol and 25-hydroxycholesterol). These studies identified a mRNA whose levels were highly induced in lipid-loaded macrophages. The mRNA encoded the human White protein, a member of the ATP-binding cassette (ABC) transporter superfamily of proteins. The mRNA levels of ABC8, the murine homolog of the human white gene, were also induced when a murine macrophage cell line, RAW264.7, was incubated with acetylated low density lipoprotein and sterols. Additional studies demonstrated that white/ABC8 mRNA levels were induced by specific oxysterols that included 25-, 20(S)-, and 22(R)-hydroxycholesterol, and by a retinoid X receptor-specific ligand. Furthermore, the oxysterol-mediated induction of ABC8 expression in mouse peritoneal macrophages was dependent on the presence of the nuclear oxysterol receptors, liver X receptors (LXRs). Macrophages derived from mice lacking both LXRalpha and LXRbeta failed to up-regulate the expression of ABC8 following incubation with 22(R)-hydroxycholesterol. Oxysterol-dependent induction of white/ABC8 mRNA was blocked by actinomycin D but not by cycloheximide treatment of cells. We conclude that the white and ABC8 genes are primary response genes that are transcriptionally activated by specific oxysterols and that this induction is mediated by the LXR subfamily of nuclear hormone receptors. These data strongly support the hypothesis that white/ABC8 has a role in cellular sterol homeostasis.

Enhancement of mdr1 gene expression in normal tissue adjacent to advanced breast cancer

In the present study, mdr1 gene expression was investigated by a sensitive reverse transcriptase-PCR assay in advanced breast cancer and in corresponding adjacent normal tissues obtained before and after treatment with primary chemotherapy. Comparatively to normal tissues, a significant induction of mdr1 expression was observed in untreated tumors (p = 0.0222). Similarly, a significant induction of mdr1 expression was revealed when treated samples were compared to untreated counterparts (p = 0.0222), but no differences were detected between tumor and normal samples (p = 0.3199). Noteworthy, a significant induction of mdr1 gene expression occurred in treated normal samples comparatively to untreated ones (p = 0.0037), and this induction was even more important in normal than in tumoral tissue (p = 0.0627). However, neither the basal expression nor the induction of mdr1 were correlated with subsequent response to chemotherapy or with survival. Thus, in agreement with previous reports, our data show that chemotherapy induce mdr1 gene expression in breast cancer cells, but they also indicate that a similar phenomenon occurs in adjacent normal tissues. Therefore, our results strongly suggest that mdr1 gene overexpression is not a characteristic of breast malignant cells, but rather constitutes a general phenomenon occurring both in normal and tumor cells which could explain at least in part the absence of relationship between mdr1 expression and the clinical outcome of breast cancer patients.

Lipid traffic: the ABC of transbilayer movement

Membrane lipids do not spontaneously exchange between the two leaflets of lipid bilayers because the polar headgroups cannot cross the hydrophobic membrane interior. Cellular membranes, notably eukaryotic plasma membranes, are equipped with special proteins that actively translocate lipids from one leaflet to the other. In addition, cellular membranes contain proteins that facilitate a passive equilibration of lipids between the two membrane halves. In recent years, a growing number of proteins have been put forward as lipid translocators or facilitators. Unexpectedly, some of these appear to be required for efficient translocation of lipids lacking bulky headgroups, like cholesterol and fatty acids. The candidate lipid translocators identified so far belong to large protein families whose other members include pumps for amphiphilic molecules like bile salts and drugs.

P-glycoprotein is localized in caveolae in resistant cells and in brain capillaries

A significant proportion of P-glycoprotein (P-gp) and caveolin was co-localized in caveolae isolated from resistant (CH(R)C5) cells overexpressing P-gp and from drug-sensitive Chinese hamster ovary cells (AuxB1). The proportion of P-gp and caveolin associated with caveolar microdomains was higher in CH(R)C5 cells grown in the presence of P-gp substrates (cyclosporin A or colchicine) than in untreated CH(R)C5 cells. Coimmunoprecipitation of P-gp and caveolin from CH(R)C5 lysates suggests that there is a physical interaction between them. Furthermore, co-localization of P-gp and caveolin was found in caveolae from brain capillaries, indicating that this association also takes place in vivo.

Overexpression of MDR1 in an intestinal cell line results in increased cholesterol uptake from micelles

The multiple drug resistance protein, MDR1, is highly expressed on the apical surface of intestinal epithelial cells. The physiologic substrate of this protein remains unclear. Several studies using compounds known to act as MDR1 inhibitors have suggested that MDR1 may be involved in the transport of cholesterol from the plasma membrane to the endoplasmic reticulum where it is esterified. To examine the role of MDR1 in cholesterol uptake by intestinal cells, the rat intestinal epithelial cell line IEC-18, was stably transfected with human MDR1. MDR1-transfected cells exhibited increased expression of MDR1 protein, reduced accumulation of vinblastine and increased uptake of [(3)H]cholesterol from cholesterol/monolein/taurocholate micelles. These studies provide the first direct evidence that the level of MDR1 expression in intestinal cells can influence the amount of cholesterol taken up by those cells. This is also the first demonstration that a multiple drug resistance protein can function in the net uptake, rather than efflux, of a substrate.

Choroid plexus epithelial expression of MDR1 P glycoprotein and multidrug resistance-associated protein contribute to the blood-cerebrospinal-fluid drug-permeability barrier

The blood-brain barrier and a blood-cerebrospinal-fluid (CSF) barrier function together to isolate the brain from circulating drugs, toxins, and xenobiotics. The blood-CSF drug-permeability barrier is localized to the epithelium of the choroid plexus (CP). However, the molecular mechanisms regulating drug permeability across the CP epithelium are defined poorly. Herein, we describe a drug-permeability barrier in human and rodent CP mediated by epithelial-specific expression of the MDR1 (multidrug resistance) P glycoprotein (Pgp) and the multidrug resistance-associated protein (MRP). Noninvasive single-photon-emission computed tomography with 99mTc-sestamibi, a membrane-permeant radiopharmaceutical whose transport is mediated by both Pgp and MRP, shows a large blood-to-CSF concentration gradient across intact CP epithelium in humans in vivo. In rats, pharmacokinetic analysis with 99mTc-sestamibi determined the concentration gradient to be greater than 100-fold. In membrane fractions of isolated native CP from rat, mouse, and human, the 170-kDa Pgp and 190-kDa MRP are identified readily. Furthermore, the murine proteins are absent in CP isolated from their respective mdr1a/1b(-/-) and mrp(-/-) gene knockout littermates. As determined by immunohistochemical and drug-transport analysis of native CP and polarized epithelial cell cultures derived from neonatal rat CP, Pgp localizes subapically, conferring an apical-to-basal transepithelial permeation barrier to radiolabeled drugs. Conversely, MRP localizes basolaterally, conferring an opposing basal-to-apical drug-permeation barrier. Together, these transporters may coordinate secretion and reabsorption of natural product substrates and therapeutic drugs, including chemotherapeutic agents, antipsychotics, and HIV protease inhibitors, into and out of the central nervous system.