Cholesterol modulates P-glycoprotein activity in human peripheral blood mononuclear cells

SIAH1 ubiquitination-modified HMGCR inhibits lung cancer progression and promotes drug sensitivity through cholesterol synthesis

BACKGROUNDS

Lung cancer is one of the most frequently diagnosed cancers and the leading cause of cancer-related deaths worldwide. Deep understanding of chemoresistance will lead to remarkable progress in lung cancer treatment strategy. Cholesterol accumulation was associated with cisplatin resistance in lung cancer treatment. And we found the degree of cisplatin resistance was correlated with the expression of the cholesterol synthesis HMGCR.

METHODS

We analyzed a group of 42 lung cancer patients who received cisplatin treatment after lung resection surgery. The expression of HMGCR and its correlation with cholesterol in lung cancer cell lines were determined by qRT-PCR and ELISA analyses. We focus on the function and mechanism of HMGCR in lung cancer and reveal that knockdown of HMGCR expression inhibits the proliferation, colony formation, and migration of lung cancer cell lines in vitro or in vivo and dramatically enhances the efficacy of cisplatin.

RESULTS

Through mechanism studies, we illustrate that SIAH1, an E3 ubiquitin-protein ligase, ubiquitination modifies HMGCR and inhibits efflux protein activity via regulating cholesterol synthesis. In vivo experiments showed that SIAH1 overexpression or using HMGCR knockdown retard tumor growth and enhanced the efficacy of cisplatin. In summary, HMGCR affects cholesterol metabolism by regulating key enzymes in cholesterol synthesis, thereby reducing drug sensitivity.

CONCLUSION

This study indicates that lung cancer patients with lower HMGCR levels may lead to a better prognosis and provide a potential treatment by SIAH1 overexpression for lung cancer patients with cisplatin resistance.

Bexarotene Promotes Cholesterol Efflux and Restricts Apical-to-Basolateral Transport of Amyloid-β Peptides in an In Vitro Model of the Human Blood-Brain Barrier

One of the prime features of Alzheimer's disease (AD) is the excessive accumulation of amyloid-β (Aβ) peptides in the brain. Several recent studies suggest that this phenomenon results from the dysregulation of cholesterol homeostasis in the brain and impaired bidirectional Aβ exchange between blood and brain. These mechanisms appear to be closely related and are controlled by the blood-brain barrier (BBB) at the brain microvessel level. In animal models of AD, the anticancer drug bexarotene (a retinoid X receptor agonist) has been found to restore cognitive functions and decrease the brain amyloid burden by regulating cholesterol homeostasis. However, the drug's therapeutic effect is subject to debate and the exact mechanism of action has not been characterized. Therefore, the objective of this present study was to determine bexarotene's effects on the BBB. Using an in vitro model of the human BBB, we investigated the drug's effects on cholesterol exchange between abluminal and luminal compartments and the apical-to-basolateral transport of Aβ peptides across the BBB. Our results demonstrated that bexarotene induces the expression of ABCA1 but not ApoE. This upregulation correlates with an increase in ApoE2-, ApoE4-, ApoA-I-, and HDL-mediated cholesterol efflux. Regarding the transport of Aβ peptides, bexarotene increases the expression of ABCB1, which in turn decreases Aβ apical-to-basolateral transport. Our results showed that bexarotene not only promotes the cholesterol exchange between the brain and the blood but also decreases the influx of Aβ peptides across BBB, suggesting that bexarotene is a promising drug candidate for the treatment of AD.

Drug transporters in tissues and cells relevant to sexual transmission of HIV: Implications for drug delivery

Efflux and uptake transporters of drugs are key regulators of the pharmacokinetics of many antiretroviral drugs. A growing body of literature has revealed the expression and functionality of multiple transporters in female genital tract (FGT), colorectal tissue, and immune cells. Drug transporters could play a significant role in the efficacy of preventative strategies for HIV-1 acquisition. Pre-exposure prophylaxis (PrEP) is a promising strategy, which utilizes topically (vaginally or rectally), orally or other systemically administered antiretroviral drugs to prevent the sexual transmission of HIV to receptive partners. The drug concentration in the receptive mucosal tissues and target immune cells for HIV is critical for PrEP effectiveness. Hence, there is an emerging interest in utilizing transporter information to explain tissue disposition patterns of PrEP drugs, to interpret inter-individual variability in PrEP drug pharmacokinetics and effectiveness, and to improve tissue drug exposure through modulation of the cervicovaginal, colorectal, or immune cell transporters. In this review, the existing literature on transporter expression, functionality and regulation in the transmission-related tissues and cells is summarized. In addition, the relevance of transporter function for drug delivery and strategies that could exploit transporters for increased drug concentration at target locales is discussed. The overall goal is to facilitate an understanding of drug transporters for PrEP optimization.

Recent advances in the exploration of the bile salt export pump (BSEP/ABCB11) function

INTRODUCTION

The bile salt export pump (BSEP/ABCB11), residing in the apical membrane of hepatocyte, mediates the secretion of bile salts into the bile. A range of human diseases is associated with the malfunction of BSEP, including fatal hereditary liver disorders and mild cholestatic conditions. Manifestation of these diseases primarily depends on the mutation type; however, other factors such as hormonal changes and drug interactions can also trigger or influence the related diseases.

AREAS COVERED

Here, we summarize the recent knowledge on BSEP by covering its transport properties, cellular localization, regulation and major mutations/polymorphisms, as well as the hereditary and acquired diseases associated with BSEP dysfunction. We discuss the different model expression systems employed to understand the function of the BSEP variants, their drug interactions and the contemporary therapeutic interventions.

EXPERT OPINION

The limitations of the available model expression systems for BSEP result in controversial conclusions, and obstruct our deeper insight into BSEP deficiencies and BSEP-related drug interactions. The knowledge originating from different methodologies, such as clinical studies, molecular genetics, as well as in vitro and in silico modeling, should be integrated and harmonized. Increasing availability of robust molecular biological tools and our better understanding of the mechanism of BSEP deficiencies should make the personalized, mutation-based therapeutic interventions more attainable.

Expression and activity of ABCG2, but not ABCB1 or OATP1B1, are associated with cholesterol levels: evidence from in vitro and in vivo experiments

Aim: This study investigated whether cholesterol levels influence the expression and function of drug transporters and whether statin treatments could alter this by reducing plasma low-density lipoprotein cholesterol levels. Patients & methods: The mRNA expression and function of OATP1B1, ABCB1 and ABCG2 were assessed in peripheral blood mononuclear cells (PBMCs) of healthy subjects and from patients with familial hypercholesterolemia (FH) before and after statin treatment by real-time PCR and flow cytometric assay, respectively. The effects of statin exposure and cholesterol depletion in PBMCs and in cell lines were assessed. Results: ABCG2 expression and activity in PBMCs in patients with FH were 2-fold and 26-fold higher, respectively, than those of the healthy subjects (p < 0.001 for both). Statin treatment decreased ABCG2 expression and function in patients with FH. Depletion of cholesterol ex vivo reduced ABCG2 expression in PBMCs and reduced ABCG2 activity in liver and colon cells. Conclusion: This study suggested that statins may downregulate ABCG2 expression and function through reducing low-density lipoprotein cholesterol levels.

Original submitted 25 November 2013; Revision submitted 21 March 2014

Interrelationship between ATP-binding cassette transporters and oxysterols

ATP-binding cassette (ABC) transporters constitute a ubiquitous superfamily of membrane proteins responsible for the translocation of several substances across membranes using the chemical energy provided by ATP hydrolysis. ABC transporters participate in many physiological and pathophysiological processes, including cholesterol and lipid transportation and multidrug resistance. Oxysterols are the products of cholesterol oxidation, formed by both enzymatic and non-enzymatic mechanisms. The role of oxysterols in cholesterol metabolism and several diseases has been widely investigated, but many questions remain to be answered. Several lines of evidence link ABC transporter functions with cholesterol and oxysterol metabolism. This review discusses ABC transporters, oxysterols, and how they interact with each other.

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.

ATP-binding cassette transporter G2 activity in the bovine spermatozoa is modulated along the epididymal duct and at ejaculation

During their epididymal maturation, stabilizing factors such as cholesterol sulfate are associated with the sperm plasma membrane. Cholesterol is sulfated in epididymal spermatozoa by the enzyme estrogen sulfotransferase. Because of its role in the efflux of sulfate conjugates formed intracellularly by sulfotransferases, the ATP-binding cassette membrane transporter G2 (ABCG2) might have a role in the translocation of this compound across the plasma membrane. In the present study we showed that ABCG2 is present in the plasma membrane overlaying the acrosomal region of spermatozoa recovered from testis, epididymis, and after ejaculation. Although ABCG2 is also present in epididymosomes, the transporter is not transferred to spermatozoa via this mechanism. Furthermore, although epididymal sperm ABCG2 was shown to be functional, as determined by its ability to extrude Hoechst 33342 in the presence of the specific inhibitor Fumitremorgin C, ABCG2 present in ejaculated sperm was found to be nonfunctional. Additional experiments demonstrated that phosphorylation of ABCG2 tyrosyl residues, but not its localization in lipid rafts, is the mechanism responsible for its functionality. Dephosphorylation of ABCG2 in ejaculated spermatozoa is proposed to cause a partial protein relocalization to other intracellular compartments. Prostasomes are proposed to have a role in this process because incubation with this fraction of seminal plasma induces a decrease in the amount of ABCG2 in the associated sperm membrane fraction. These results demonstrate that ABCG2 plays a role in epididymal sperm maturation, but not after ejaculation. The loss of ABCG2 function after ejaculation is proposed to be regulated by prostasomes.

Structure of ABC transporters

ABC (ATP-binding cassette) transporters are primary active membrane proteins that translocate solutes (allocrites) across lipid bilayers. The prototypical ABC transporter consists of four domains: two cytoplasmic NBDs (nucleotide-binding domains) and two TMDs (transmembrane domains). The NBDs, whose primary sequence is highly conserved throughout the superfamily, bind and hydrolyse ATP to power the transport cycle. The TMDs, whose primary sequence and protein fold can be quite disparate, form the translocation pathway across the membrane and generally (but not always) determine allocrite specificity. Structure determination of ABC proteins initially took advantage of the relative ease of expression and crystallization of the hydrophilic bacterial NBDs in isolation from the transporter complex, and revealed detailed information on the structural fold of these domains, the amino acids involved in the binding and hydrolysis of nucleotide, and the head-to-tail arrangement of the NBD-NBD dimer interface. More recently, several intact transporters have been crystallized and three types have, so far, been characterized: type I and II ABC importers, and ABC exporters. All three are present in prokaryotes, but only the ABC exporters appear to be present in eukaryotes. Their structural determination has provided insight into the mechanisms of energy and signal transduction between the NBDs and TMDs (i.e. between the ATP- and allocrite-binding sites) and, for some, the nature of the allocrite-binding site(s) within the TMDs. In this chapter, we focus primarily on the ABC exporters and describe the structural, biochemical and biophysical evidence for and against the controversial bellows-like mechanism proposed for allocrite efflux.

Pegylated phosphotidylethanolamine inhibiting P-glycoprotein expression and enhancing retention of doxorubicin in MCF7/ADR cells

The failure of the clinical treatment of cancer patients is often attributed to drug resistance of the tumor to chemotherapeutic agents. P-glycoprotein (P-gp) contributes to drug resistance via adenosine 5'-triphosphate (ATP)-dependent drug efflux pumps and is widely expressed in many human cancers. Up to date, a few of nanomaterials have shown the effects on P-gp function by different ways. To study the mechanism of the increased cytotoxicity of doxorubicin (DOX) by pegylated phosphotidylethanolamine (PEG-PE) in drug-resistant cancer cells, a series of in vitro cell assays were performed, including identification of P-gp function, quantitative studies on uptake and efflux of DOX, inhibitory effects of blank PEG-PE micelles on mRNA and protein levels of P-gp, and intracellular ATP content alteration. Finally, combining MDR-1 RNA interference (siRNA) with DOX encapsulated in PEG-PE micelles (M-DOX) to improve cytotoxicity of DOX was also studied. M-DOX showed fivefold lower the concentration that caused 50% killing tumor cell than that of free DOX in the P-gp-overexpressing MCF-7 breast cancer (MCF-7/ADR) cells. M-DOX enhanced the cellular uptake and retention of DOX in MCF-7/ADR cells. PEG-PE block molecules can inhibit P-gp expression through downregulating MDR-1 gene. Cytotoxicity of M-DOX was further improved by knocking down the MDR-1 gene using siRNA in the multidrug-resistant cells. We conclude that the increased cytotoxicity of DOX encapsulated in PEG-PE micelle is due to the reduced P-gp expression by PEG-PE block molecules, and accordingly enhancing the cellular accumulation of DOX. To overcome drug resistance of tumor cells, the combination of nanotechnology and biotechnology could be an effective strategy such as PEG-PE formed micelles and siRNA.

Association of ABCB1 gene polymorphisms with plasma lipid and apolipoprotein concentrations in the STANISLAS cohort

BACKGROUND

While involvement of ABCB1 is well known in drug transport, its metabolite transport role is not so well understood. Like other ABC transporters, ABCB1 might be implicated in cholesterol homeostasis and ABCB1 polymorphisms which are responsible for drug resistance might affect lipid homeostasis. Our objective was thus to investigate the implication of ABCB1 polymorphisms and haplotypes in the genetic variability of lipid constituents in healthy people.

METHODS

T-129C, G-1A, A61G, G1199A, C1236T, T-76A, G2677T/A and C3435T polymorphisms were genotyped in 371 supposed healthy individuals from the STANISLAS cohort. Each polymorphism was tested with plasma concentrations of total cholesterol, HDL and LDL cholesterol, triglycerides and apolipoproteins A1, B, C3 and E.

RESULTS

After adjustment for covariates, carriers of at least one 3435T allele had a significant higher level of apolipoprotein A1 (p = 0.005). In addition, significant correlations were observed in a sex-dependent manner. Women carrying either T-76 or 1236T allele (tendency with G-1 and 2677T/A) had lower total cholesterol (p < or = 0.01) and apolipoprotein B (T-76 exclusively, p=0.002). Haplotypes analysis was not more informative than the single polymorphisms except G2677T/A-C3435T haplotypes for apolipoprotein A1 concentration.

CONCLUSION

ABCB1 polymorphisms contribute to the genetic variability of plasma values of lipids and lipoproteins in healthy people.

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.

Mechanism of multidrug recognition by MDR1/ABCB1

MDR1/ABCB1, a member of the ABC group of proteins, is clinically important because it is not only involved in multidrug resistance in cancer but also affects the pharmacokinetic properties of various drugs. The most puzzling feature of MDR1 is that it recognizes and transports such a wide variety of substrates. In the present review, the function of MDR1 is compared with that of other ABC proteins, particularly MDR2/ABCB4, to understand the mechanism of drug recognition and transport by MDR1. MDR2, the amino acid sequence of which has 86% similarity to that of MDR1, excretes phosphatidylcholine and cholesterol in the presence of bile salts. ABCA1 transfers phospholipids, preferentially phosphatidylcholine, and cholesterol to lipid-free apoA-I to generate pre-beta-HDL, and ABCG1 excretes phospholipids, preferentially sphingomyelin, and cholesterol. Cholesterol also binds directly to MDR1 and modulates substrate recognition by MDR1. Cholesterol may fill the empty space of the drug-binding site and aid the recognition of small drugs, and facilitates the ability of MDR1 to recognize compounds with various structures and molecular weights. Eukaryote ABC proteins may retain similar substrate binding pockets and move bound substrates in an ATP-dependent manner. The prototype of eukaryote ABC proteins might be those involved in membrane lipid transport.

Plasma LDL cholesterol has no impact on P-glycoprotein (MDR1/ABCB1) activity in human peripheral blood mononuclear cells

Several studies have demonstrated that the adenosine triphosphate-binding cassette transporter P-glycoprotein (P-gp) is at least partly located in cholesterol- and sphingolipid-enriched parts of the plasma membrane called "lipid rafts" and that modification of cellular cholesterol content has an impact on the activity of P-gp in vitro and ex vivo. Cholesterol modulation in vitro does not closely reflect the in vivo situation. The aim of our study was therefore to investigate whether differences in individual plasma low-density lipoprotein (LDL) cholesterol levels in humans have an impact on cholesterol content in peripheral blood mononuclear cells (PBMCs) and thereby on individual activity of P-gp. PBMCs of 20 ambulatory patients with elevated LDL cholesterol (173.9 +/- 22.4 mg/dl; range 151.0-234.4 mg/dl) and 28 controls (125.2 +/- 16.9 mg/dl; range 74.6-149.6 mg/dl) were isolated. Cellular cholesterol was measured by an enzymatic fluorimetric assay, efflux activity of P-gp in PBMCs was determined by a flow cytometric method (rhodamine123 efflux), and messenger ribonucleic acid expression was quantified by reverse transcriptase real-time polymerase chain reaction (RT-PCR). There was no difference in cellular cholesterol or P-gp activity between the two groups suggesting that high plasma LDL cholesterol concentration as observed in dyslipidemic patients does not correlate with cellular cholesterol content or P-gp activity in PBMCs. There was, however, a significant negative relationship between age and P-gp efflux activity indicating that P-gp activity in PBMCs decreases with advancing age. These results need further confirmation because investigation of age dependency of P-gp activity was not the primary aim of the study.

Localization of the human breast cancer resistance protein (BCRP/ABCG2) in lipid rafts/caveolae and modulation of its activity by cholesterol in vitro

Breast cancer resistance protein (BCRP/ABCG2) is an active efflux pump that belongs to the ATP-binding cassette (ABC) transporter family. It is located in various tissues involved in drug absorption, distribution, and elimination and plays an important role in multidrug resistance. For P-glycoprotein, another member of the ABC transporter family, it is well established that it is at least partly located in cholesterol and sphingolipid-enriched domains of the plasma membrane called "lipid rafts" and that the composition of the membrane lipids may modulate its efflux activity. This study addressed the compartmentalization of BCRP in the plasma membrane and the influence of membrane cholesterol on the efflux activity of BCRP. As a cell model, we used the canine kidney epithelial cell line MDCKII-BCRP transfected with the cDNA encoding human BCRP and the corresponding parental cell line MDCKII. Cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) provoked a 40% decrease in BCRP activity (p < 0.01) assessed with flow cytometry (pheophorbide A efflux assay). Cholesterol repletion with MbetaCD/cholesterol-inclusion complexes restored BCRP function, and cholesterol saturation of native cells did not further enhance BCRP activity. Coimmunoprecipitation experiments indicated a physical interaction between BCRP and caveolin-1, and Western blot analysis after density gradient ultracentrifugation demonstrated that BCRP is located in detergent-resistant membranes that also contain caveolin-1. In conclusion, our results demonstrate for the first time that BCRP is located in membrane rafts and that cholesterol has impact on its efflux activity.

Multidrug resistance protein 1 is not associated to detergent-resistant membranes

Multidrug resistance protein 1 (MRP1) is a member of the ATP-binding cassette superfamily. Using the energy provided by ATP hydrolysis, it transports a broad spectrum of substrates across the plasma membrane, including hormones, leukotriene C(4), bile salts, and anti-cancer drugs. Recent works have suggested that P-glycoprotein is associated to cholesterol and sphingolipid-rich membrane microdomains and that cholesterol upregulates its ATPase and drug transport activities. Confocal microscopy experiments and Triton X-100 extraction of detergent-resistant membranes provide evidence that MRP1 is not located in raft-like structures and that its activity is downregulated by cholesterol. The data are discussed in terms of cholesterol-protein interaction and topology.

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.

Modulation of drug-stimulated ATPase activity of human MDR1/P-glycoprotein by cholesterol

MDR1 (multidrug resistance 1)/P-glycoprotein is an ATP-driven transporter which excretes a wide variety of structurally unrelated hydrophobic compounds from cells. It is suggested that drugs bind to MDR1 directly from the lipid bilayer and that cholesterol in the bilayer also interacts with MDR1. However, the effects of cholesterol on drug-MDR1 interactions are still unclear. To examine these effects, human MDR1 was expressed in insect cells and purified. The purified MDR1 protein was reconstituted in proteoliposomes containing various concentrations of cholesterol and enzymatic parameters of drug-stimulated ATPase were compared. Cholesterol directly binds to purified MDR1 in a detergent soluble form and the effects of cholesterol on drug-stimulated ATPase activity differ from one drug to another. The effects of cholesterol on K(m) values of drug-stimulated ATPase activity were strongly correlated with the molecular mass of that drug. Cholesterol increases the binding affinity of small drugs (molecular mass <500 Da), but does not affect that of drugs with a molecular mass of between 800 and 900 Da, and suppresses that of valinomycin (molecular mass >1000 Da). V(max) values for rhodamine B and paclitaxel are also increased by cholesterol, suggesting that cholesterol affects turnover as well as drug binding. Paclitaxel-stimulated ATPase activity of MDR1 is enhanced in the presence of stigmasterol, sitosterol and campesterol, as well as cholesterol, but not ergosterol. These results suggest that the drug-binding site of MDR1 may best fit drugs with a molecular mass of between 800 and 900 Da, and that cholesterol may support the recognition of smaller drugs by adjusting the drug-binding site and play an important role in the function of MDR1.

Lipid rafts: new tools and a new component

Lipid rafts are liquid ordered membrane domains enriched with sphingolipids and cholesterol. After 20 years since the proposal of the original concept, the structure and function of lipid rafts are still obscure. Recently new tools to study lipid rafts have been developed. Lysenin is a sphingomyelin binding protein that specifically recognizes the lipid clusters. Poly(ethyleneglycol)-derivatized cholesterol ether (PEG-Chol) is a non-toxic cholesterol probe. These probes have revealed the heterogeneity of lipid rafts. The heterogeneity of lipid rafts is further supported by the discovery of a new lipid component, phosphatidylglucoside. Metabolic inhibitors are another useful tool. Sulfamisterin is a new addition to the serine palmitoyltransferase inhibitors. Recent findings have uncovered a previously unrecognized activity of a glycosphingolipid synthesis inhibitor, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP).

Atp8b1 deficiency in mice reduces resistance of the canalicular membrane to hydrophobic bile salts and impairs bile salt transport

Progressive familial intrahepatic cholestasis type 1 (PFIC1, Byler disease, OMIM 211600) is a severe inherited liver disease caused by mutations in ATP8B1. ATP8B1 is a member of the type 4 subfamily of P-type ATPases, which are phospholipid flippases. PFIC1 patients generally develop end-stage liver disease before the second decade of life. The disease is characterized by impaired biliary bile salt excretion, but the mechanism whereby impaired ATP8B1 function results in cholestasis is unclear. In a mouse model for PFIC1, we observed decreased resistance of the hepatocanalicular membrane to hydrophobic bile salts as evidenced by enhanced biliary recovery of phosphatidylserine, cholesterol, and ectoenzymes. In liver specimens from PFIC1 patients, but not in those from control subjects, ectoenzyme expression at the canalicular membrane was markedly deficient. In isolated mouse livers Atp8b1 deficiency impaired the transport of hydrophobic bile salts into bile. In conclusion, our study shows that Atp8b1 deficiency causes loss of canalicular phospholipid membrane asymmetry that in turn renders the canalicular membrane less resistant toward hydrophobic bile salts. The loss of phospholipid asymmetry may subsequently impair bile salt transport and cause cholestasis.

Differential in vivo sensitivity to inhibition of P-glycoprotein located in lymphocytes, testes, and the blood-brain barrier

A major functional component of the blood-brain barrier is P-glycoprotein. In principle, inhibition of this efflux transporter would permit greater distribution of its substrates into the brain and increased central effects. Tariquidar and elacridar, potent and selective P-glycoprotein inhibitors, were investigated in this regard using the opioid loperamide as an in vivo probe in mice. Pretreatment with both inhibitors converted intravenous loperamide from a drug without central effects to one producing antinociception. Radiolabeled loperamide tissue distribution studies indicated that inhibition was associated with increased uptake into brain and testes in the absence of changes in plasma levels, along with enhanced efflux of rhodamine 123 from CD3e+ T-lymphocytes. However, with tariquidar, the loperamide dose-response curves for testes/plasma and brain/plasma concentration ratios were shifted 6- (p = 0.07) and 25-fold (p < 0.01) to the right, respectively (ED50 = 1.48 and 5.65 mg/kg), compared with the rhodamine 123 efflux curve (ED50 0.25 mg/kg). Less pronounced shifts were noted with elacridar where the brain/plasma ratio was shifted only 2-fold relative to the rhodamine 123 efflux data (ED50 = 2.36 versus 1.34 mg/kg, respectively; p 0.01). These results indicate that the P-glycoprotein localized in the blood-brain barrier and, to a lesser extent, the testes-blood barrier is more resistant to inhibition than at other tissue sites such as the lymphocyte; moreover, the extent of this effect depends on the inhibitor. Such resistance can be overcome by a sufficiently high dose of an inhibitor; however, whether this is safely attainable in the clinical situation remains to be determined.

MDR1 Gene Polymorphism in Japanese Patients with Schizophrenia and Mood Disorders Including Depression

P-Glycoprotein (ABCB1-type P-gp), a membrane protein encoded by the multi drug resistant gene (MDR1), expressing on the blood brain barrier protects the brain from many drugs including dexamethasone. Psychiatric disorders, schizophrenia and depression, have known to have abnormal hypothalamus-pituitary-adrenal (HPA) activity, which is assessed by non-suppression of cortisol in dexamethasone suppression test. The poor response to dexamethasone in these patients' population suggested the impaired activity on dexamethasone penetration into the brain via P-gp, which was associated with MDR1 polymorphisms. We, therefore, examined five SNPs of the MDR1 gene, -1517 T>C (promoter), -41 A>G (intron -1), -129 T>C (exon 1b), 2677 G>A,T (exon 21) and 3435 C>T (exon 26), in Japanese patients with schizophrenia (n=121) and mood disorders (n=62), and compared with the control subjects (n=160). The frequency of MDR1 mutant alleles at -1517, -41 and -129 in patients with mood disorders was significantly lower (2.4, 5.6, 2.4%, respectively) than those of controls (7.8, 13.7, 7.8%, respectively) (p<0.05). The frequencies of MDR1 2677 G/A and A/A genotype in mood disorders was significantly higher (17.7, 6.5%, respectively) than controls (11.2, 0%, respectively) (p<0.05). The 2677A allele frequency in mood disorders (20.2%) was significant higher than controls (10.9%) (p<0.05). Haplotype of 129-2677-3435 (T-A-C) in mood disorders was significantly higher (14.4%) than controls (8.0%) (p<0.05). There was no significant difference in allele and genotype frequencies between the patients with schizophrenia and controls. These findings suggested that predispose to mood disorders, not schizophrenia, was associated with possible alteration of P-gp activities corresponding MDR1 polymorphism at least partly.

Alterations of lymphocyte membranes during HIV-1 infection via multiple and simultaneous entry strategies

Human immunodeficiency virus type 1 (HIV-1) must bind to and enter lymphocytes to replicate and cause the acquired immunodeficiency syndrome. The association of viral particles with the lymphocyte plasma membrane may vary according to a multitude of unknown variables, including lymphocyte membrane receptor mobilization, lipid raft aggregation, clathrin, caveolin, endosomes, microendosome-mediated penetration or penetration through a hole in the membrane. The time course of this delivery appears to be short. Fusion of the virion membrane and lymphocyte plasma membrane leads to destabilization of the lymphocyte membrane. Five morphological stages of membrane alteration were observed in the infected lymphocytes: (1) swelling, (2) splitting, (3) fusion, (4) breaking, and (5) thinning of the lipid bilayer. These plasma membrane alterations were not contributed by fixation artifacts, because the dimensions and distance between the subunits of the surface glycoprotein (SU, gp120) and the transmembrane glycoprotein (gp41) of the viral particles adjacent to the infected cells and processed at the same time remained unchanged. Destabilization of lipid raft patches in the lymphocyte plasma membrane by unknown variables may facilitate HIV-1 penetration of lymphocyte, and other cell types. This a combined review of the pertinent literature with our data showing that HIV-1 may take advantage of multiple penetration approaches simultaneously in the same cell type (H9) to overwhelm the infected cells. The ultrastructural details of H9 cultured cells infected in vitro with HIV-1 contribute to our understanding of viral particle association with the plasma membrane of infected cells.

Multidrug-resistant cancer cells contain two populations of P-glycoprotein with differently stimulated P-gp ATPase activities: evidence from atomic force microscopy and biochemical analysis

Considerable interest exists about the localization of P-gp (P-glycoprotein) in DRMs (detergent-resistant membranes) of multidrug resistant cancer cells, in particular concerning the potential modulating role of the closely related lipids and proteins on P-gp activity. Our observation of the opposite effect of verapamil on P-gp ATPase activity from DRM and solubilized-membrane fractions of CEM-resistant leukaemia cells, and results from Langmuir experiments on membrane monolayers from resistant CEM cells, strongly suggest that two functional populations of P-gp exist. The first is located in DRM regions: it displays its optimal P-gp ATPase activity, which is almost completely inhibited by orthovanadate and activated by verapamil. The second is located elsewhere in the membrane; it displays a lower P-gp ATPase activity that is less sensitive to orthovanadate and is inhibited by verapamil. A 40% cholesterol depletion of DRM caused the loss of 52% of the P-gp ATPase activity. Cholesterol repletion allowed recovery of the initial P-gp ATPase activity. In contrast, in the solubilized-membrane-containing fractions, cholesterol depletion and repletion had no effect on the P-gp ATPase activity whereas up to 100% saturation with cholesterol induced a 58% increased P-gp ATPase activity, while no significant modification was observed for the DRM-enriched fraction. DRMs were analysed by atomic force microscopy: 40-60% cholesterol depletion was necessary to remove P-gp from DRMs. In conclusion, P-gp in DRMs appears to contain closely surrounding cholesterol that can stimulate P-gp ATPase activity to its optimal value, whereas cholesterol in the second population seems deprived of this function.

Expression of the drug transporters MDR1/ABCB1, MRP1/ABCC1, MRP2/ABCC2, BCRP/ABCG2, and PXR in peripheral blood mononuclear cells and their relationship with the expression in intestine and liver

ATP binding cassette (ABC)-transporters like P-glycoprotein (multidrug resistance (MDR)1/ABCB1), the multidrug resistance associated proteins 1 and 2 (MRP1/ABCC1 and MRP2/ABCC2), and the breast cancer resistance protein (BCRP/ABCG2) have a large impact on the pharmacokinetics of numerous drugs and may also modulate the effectiveness of drug therapy. Prediction of a patient's susceptibility to xenobiotics and individualization of drug therapy would become possible, if a simple test were available for an easy screening of transporter expression. This study quantified the mRNA expression of the four ABC-transporters and of the pregnane X receptor (PXR), a key regulator in drug metabolism and efflux, in peripheral blood mononuclear cells (PBMCs), and corresponding liver or small intestine samples of humans by real-time reverse transcription-polymerase chain reaction (RT-PCR). The results obtained prove the absence of a correlation between the expression of four major ABC-transporters in PBMCs and in the intestine or liver. For all transporters (except MRP1/ABCC1 in the intestine), mRNA amount of the ABC-transporters was positively correlated with PXR expression in PBMCs and intestine. In conclusion, the study suggests that basal expression levels of the transporters are directly influenced by PXR expression in liver and PBMCs and demonstrates that PBMCs do not qualify as surrogate tissue for the expression of the four ABC-transporters in small intestine and liver. However, the transporter status in PBMCs remains important for drugs, whose primary site of therapeutic action is the lymphocyte and which are known substrates of the transporters.

EFFECT OF THE MODULATION OF THE MEMBRANE LIPID COMPOSITION ON THE LOCALIZATION AND FUNCTION OF P-GLYCOPROTEIN IN MDR1-MDCK CELLS

Multidrug resistance (MDR) is a major obstacle in cancer therapy. It results from different mechanisms; among them is P-glycoprotein (P-gp)-mediated drug efflux out of cells. The mechanism of action remains elusive. The membrane lipid surrounding of P-gp, especially cholesterol, has been postulated to play an important role. To determine the effect of cholesterol depletion on P-gp, Madin Darby canine kidney (MDCK) cells, transfected with the mdr1 gene (MDR1-MDCK cells), were treated with methyl-beta-cyclodextrin (MbetaCD). The localization and function of P-gp were analyzed using confocal laser scanning microscopy. Treatment with 100 mM MbetaCD did not affect viability but altered the structural appearance of the cells and abolished efflux of rhodamine 123, a P-gp substrate. The MbetaCD treatment released P-gp from intact cells into the supernatant and reduced the amount of P-gp in total membrane preparations. The P-gp was shifted from the raft fractions (1% Triton X-100, 4 degrees C) to higher density fractions in MbetaCD-treated cells. The amount of cholesterol was significantly decreased in the raft fractions. Treatment of cells with 1-phenyl-2-decanoylamino-3-morpholino-1-propanol, a glucosylceramide synthase inhibitor, also led to a shift of P-gp to higher density fractions. These results show that removal of cholesterol modulates the membrane lipid composition, changes the localization of P-gp, and results in loss of P-gp function.

Modulation of cellular cholesterol alters P-glycoprotein activity in multidrug-resistant cells

The drug transporter P-glycoprotein (ABCB1) plays an important role in drug distribution and elimination, and when overexpressed it may confer multidrug resistance (MDR). P-glycoprotein is localized in the plasma membrane, especially within rafts and caveolae, characterized as detergent-resistant membranes (DRMs). This study investigated the effect of cholesterol depletion and repletion as well as saturation on subcellular localization and function of P-glycoprotein to determine the effect of DRM localization on P-glycoprotein-mediated drug efflux. In L-MDR1 overexpressing human P-glycoprotein, cholesterol depletion removed P-glycoprotein from the raft membranes into non-DRM fractions, whereas repletion fully reconstituted raft localization. P-glycoprotein function was assessed by realtime monitoring with confocal laser scanning microscopy using BODIPY-verapamil as substrate. Cholesterol depletion reduced P-glycoprotein function in L-MDR1 cells resulting in intracellular substrate accumulation (159% +/- 43, p < 0.001; control = 100%). Cholesterol repletion reduced intracellular substrate fluorescence (120% +/- 36, p < 0.001) and restored the transporter activity. Addition of surplus cholesterol (saturation) even enhanced drug efflux in L-MDR1 cells, leading to reduced intracellular accumulation of BODIPY-verapamil (69% +/- 10, p < 0.001). Transport of BODIPY-verapamil in cells not expressing human P-glycoprotein (LLC-PK1) was not susceptible to cholesterol alterations. These results demonstrate that cholesterol alterations influence P-glycoprotein localization and function, which might contribute to the large interindividual variability of P-glycoprotein activity known from in vivo studies.