P-glycoprotein-mediated colchicine resistance in different cell lines correlates with the effects of colchicine on P-glycoprotein conformation

Cognitive Status is Better in Older Adults under Colchicine Treatment: A Case-Control Study

PURPOSE

We aimed to investigate the effects of colchicine, an important anti-inflammatory agent, on cognitive functions in a geriatric population diagnosed with gout or osteoarthritis by comparing it to non-colchicine users.

METHODS

138 geriatric patients (67 colchicine users and 71 non-users) were enrolled. Within comprehensive geriatric assessment (CGA), cognitive status assessment via Mini-Mental State Examination test (MMSE), Quick Mild Cognitive Impairment Screening test (Qmci), clock drowning test (CDT), and digit span tests were performed.

RESULTS

Median age was 68 (65-72), and there were 82 female (59.4%) patients. The scores of CDT, Backward Digit Span Test, MMSE-Total, MMSE-Attention, MMSE-Motor Function, Qmci-Total Score, Qmci-Clock drawing, and Qmci-Logical Memory were significantly higher in the colchicine user group (p < .005), showing better cognitive function. Adjusted model analysis showed that colchicine usage is independently correlated with higher Qmci-Total Score and Qmci-Logical Memory Score (For Qmci total score β = 7.87 95%CI = 5.48-10.27, p = <0.0001, and for Qmci Logical memory score β = 3.52, 95%CI = 2.12-4.91, p = <0.0001).

CONCLUSION

To the best of our knowledge, this is the first study revealing that colchicine usage is associated with better cognitive performance in older adults. Further investigations with a prospective, larger-sampled and randomized design are needed to show the causal relationship between colchicine and cognition.

An insight into the anticancer potential of carbamates and thiocarbamates of 10-demethoxy-10-methylaminocolchicine

Colchicine shows very high antimitotic activity, therefore, it is used as a lead compound for generation of new anticancer agents. In the hope of developing novel, useful drugs with more favourable pharmacological profiles, a series of doubly modified colchicine derivatives has been designed, synthesized and characterized. These novel carbamate or thiocarbamate derivatives of 10-demethoxy-10-methylaminocolchicine have been tested for their antiproliferative activity against four human cancer cell lines. Additionally, their mode of action has been evaluated as colchicine binding site inhibitors, using molecular docking studies. Most of the tested compounds showed greater cytotoxicity (IC₅₀ in a low nanomolar range) and were characterized by a higher selectivity index than standard chemotherapeutics such as cisplatin and doxorubicin as well as unmodified colchicine. Their pharmacological use in cancer therapy could possibly be accomplished with lower dosages and result in less acute toxicity problems than in the case of colchicine. In addition, we present a QSAR model for predicting the antiproliferative activity of doubly modified derivatives for two tumour cell lines.

Human ABCB1 with an ABCB11-like degenerate nucleotide binding site maintains transport activity by avoiding nucleotide occlusion

Several ABC exporters carry a degenerate nucleotide binding site (NBS) that is unable to hydrolyze ATP at a rate sufficient for sustaining transport activity. A hallmark of a degenerate NBS is the lack of the catalytic glutamate in the Walker B motif in the nucleotide binding domain (NBD). The multidrug resistance transporter ABCB1 (P-glycoprotein) has two canonical NBSs, and mutation of the catalytic glutamate E556 in NBS1 renders ABCB1 transport-incompetent. In contrast, the closely related bile salt export pump ABCB11 (BSEP), which shares 49% sequence identity with ABCB1, naturally contains a methionine in place of the catalytic glutamate. The NBD-NBD interfaces of ABCB1 and ABCB11 differ only in four residues, all within NBS1. Mutation of the catalytic glutamate in ABCB1 results in the occlusion of ATP in NBS1, leading to the arrest of the transport cycle. Here we show that despite the catalytic glutamate mutation (E556M), ABCB1 regains its ATP-dependent transport activity, when three additional diverging residues are also replaced. Molecular dynamics simulations revealed that the rescue of ATPase activity is due to the modified geometry of NBS1, resulting in a weaker interaction with ATP, which allows the quadruple mutant to evade the conformationally locked pre-hydrolytic state to proceed to ATP-driven transport. In summary, we show that ABCB1 can be transformed into an active transporter with only one functional catalytic site by preventing the formation of the ATP-locked pre-hydrolytic state in the non-canonical site.

ABC transporters are one of the largest membrane protein superfamilies, present in all organisms from archaea to humans. They transport a wide range of molecules including amino acids, sugars, vitamins, nucleotides, peptides, lipids, metabolites, antibiotics, and xenobiotics. ABC transporters energize substrate transport by hydrolyzing ATP in two symmetrically arranged nucleotide binding sites (NBSs). The human multidrug resistance transporter ABCB1 has two active NBSs, and it is generally believed that integrity and cooperation of both sites are needed for transport. Several human ABC transporters, such as the bile salt transporter ABCB11, have one degenerate NBS, which has significantly reduced ATPase activity. Interestingly, unilateral mutations affecting one of the two NBSs completely abolish the function of symmetrical ABC transporters. Here we engineered an ABCB1 variant with a degenerate, ABCB11-like NBS1, which can nevertheless transport substrates. Our results indicate that ABCB1 can mediate active transport with a single active site, questioning the validity of models assuming strictly alternating catalysis.

New Series of Double-Modified Colchicine Derivatives: Synthesis, Cytotoxic Effect and Molecular Docking

Colchicine is a well-known anticancer compound showing antimitotic effect on cells. Its high cytotoxic activity against different cancer cell lines has been demonstrated many times. In this paper we report the syntheses and spectroscopic analyses of novel colchicine derivatives obtained by structural modifications at C7 (carbon-nitrogen single bond) and C10 (methylamino group) positions. All the obtained compounds have been tested in vitro to determine their cytotoxicity toward A549, MCF-7, LoVo, LoVo/DX, and BALB/3T3 cell lines. The majority of obtained derivatives exhibited higher cytotoxicity than colchicine, doxorubicin and cisplatin against the tested cancerous cell lines. Additionally, most of the presented derivatives were able to overcome the resistance of LoVo/DX cells. Additionally, their mode of binding to β-tubulin was evaluated in silico. Molecular docking studies showed that apart from the initial amides 1 and 2, compound 14, which had the best antiproliferative activity (IC₅₀ = 0.1–1.6 nM), stood out also in terms of its predicted binding energy and probably binds best into the active site of βI-tubulin isotype.

Potential anticancer role of colchicine-based derivatives: an overview

Colchicine, the main alkaloid of the poisonous plant meadow saffron (Colchicum autumnale L.), is a classical drug used for the treatment of gout and familial Mediterranean fever. Although colchicine is not clinically used to treat cancer because of toxicity, it exerts antiproliferative effects through the inhibition of microtubule formation by blocking the cell cycle at the G2/M phase and triggering apoptosis. Colchicine can still be used as a lead compound for the generation of potential anticancer drugs. Thus, numerous analogues of colchicine have been synthesized in the hope of developing novel, useful drugs with more favourable pharmacological profiles. Several colchicine semisynthetics are less toxic than colchicine and research is being carried out on effective, less toxic colchicine semisynthetic formulations with potential drug-delivery strategies directly targeting multiple solid cancers. This review focuses on the anticancer role of some of colchicine-based derivatives and their therapeutic importance.

Toxicokinetics of Polar Chemicals in Zebrafish Embryo (Danio rerio): Influence of Physicochemical Properties and of Biological Processes

The time-resolved uptake of 17 nonionic and ionic polar compounds (logD ≤ 2) with a diversity of functional groups into zebrafish embryos (ZFE) was studied over 96 h of exposure. Among them were pharmaceuticals, pesticides and plant active ingredients. Uptake rates for the diffusion controlled passive uptake through the ZFE membrane ranged from 0.02 to 24 h(-1) for the nonionic compounds and were slower for ionic compounds (<0.008-0.08 h(-1)). The study compounds did not enrich much in the ZFE (median bioconcentration factor of 1, max. 7). Biotransformation significantly influenced the internal concentration of some of the test compounds over time (benzocaine, phenacetin, metribuzin, phenytoin, thiacloprid, valproic acid). For benzocaine, valproic acid and phenacetin several transformation products (TPs) were observed by LC-MS already at early life-stages (before 28 hpf); for benzocaine the TPs comprised >90% of the initial amount taken up into the ZFE. For six compounds internal concentrations remained very low (rel. int. conc. < 0.2). Besides biotransformation (sulfamethoxazole), poor membrane permeability (cimetidine, colchicine) and also affinity to efflux transporters (atropine and chloramphenicol) are the likely reasons for these low internal concentrations. This study outlines that the uptake of polar compounds into ZFE is influenced by their physicochemical properties. However, biological processes, biotransformation and, likely, efflux can strongly affect the internal concentrations already in early developmental stages of the ZFE. This should be considered in future toxicokinetic modeling. The evaluation of the toxicity of chemicals by ZFE requires toxicokinetic studies of the test compounds and their TPs to increase comparability to effects in fish.

Colchicine derivatives with potent anticancer activity and reduced P-glycoprotein induction liability

Colchicine (1), a nature-derived microtubule polymerization inhibitor, develops multi-drug resistance in tumor cells due to its P-gp substrate and induction activity, which in turn leads to its rapid efflux from tumor cells. This auto-induction of the efflux of colchicine remains a major challenge to medicinal chemists. Based on structure-based molecular modeling, a series of new colchicine derivatives were designed and synthesized with a potential for reduced P-gp induction liability. Screening of the prepared derivatives for P-gp induction activity revealed that a number of derivatives possess remarkably lower P-gp-induction activity (>90% intracellular accumulation of rhodamine 123 in LS-180 cells) compared to the parent natural product colchicine (62% Rh123 accumulation in LS-180 cells). The reduced P-gp-induction activity of new derivatives may be due to their reduced ability to interact and change the conformation of P-gp. The synthesized derivatives were then screened for antiproliferative activity against two colon cancer cell lines including HCT-116 and Colo-205. The derivative 4o showed potent cytotoxicity in HCT-116 cells with IC50 of 0.04 μM with significantly reduced P-gp induction liability. Compound 4o also inhibited microtubule assembly and induced expression of pro-apoptotic protein p21. In an Ehrlich solid tumor mice model, compound 4o showed 38% TGI with no mortality at 2 mg kg(-1) dose (oral). Compound 4o, with potent in vitro and in vivo anticancer activity, significantly reduced P-gp induction activity and its excellent physicochemical and pharmacokinetic properties open up new opportunities for the colchicine scaffold.

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

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

Profiles of multidrug resistance protein-1 in the peripheral blood mononuclear cells of patients with refractory epilepsy

Background

About one third of patients with epilepsy become refractory to therapy despite receiving adequate medical treatment, possibly from multidrug resistance. P-glycoprotein, encoded by multidrug resistance protein-1 (MDR1) gene, at the blood brain barrier is considered as a major factor mediating drug efflux and contributing to resistance. Given that peripheral blood mononuclear cells (PBMNCs) express MDR1, we investigated a MDR1 status of PBMNCs in various subsets of epilepsy patients and demonstrated their association with clinical characteristics.

Methodology/Principal Findings

Clinical and MDR1 data were collected from 140 patients with epilepsy, 30 healthy volunteers, and 20 control patients taking anti-epileptic drugs. PBMNCs were isolated, and basal MDR1 levels and MDR1 conformational change levels were measured by flow cytometry. MDR1 profiles were analyzed according to various clinical parameters, including seizure frequency and number of medications used in epilepsy patients. Epilepsy patients had a higher basal MDR1 level than non-epilepsy groups (p<0.01). Among epilepsy patients, there is a tendency for higher seizure frequency group to have higher basal MDR1 level (p = 0.059). The MDR1 conformational change level was significantly higher in the high-medication-use group than the low-use group (p = 0.028). Basal MDR1 (OR = 1.16 [95% CI: 1.060–1.268]) and conformational change level (OR = 1.11 [95% CI: 1.02–1.20]) were independent predictors for seizure frequency and number of medications, respectively.

Conclusions/Significance

The MDR1 profile of PBMNCs is associated with seizure frequency and medication conditions in patients with epilepsy.

Single-cell tracking with a reversing flow cytometer

We have developed an instrument based on a flow cytometer platform that is capable of tracking individual, suspended cells over extended time periods. The instrument repeatedly moves in a capillary the same volume segment of fluid containing tens to hundreds of suspended cells through the focal point of a laser. Individual cells are then tracked based on the timing of when they cross the laser, and cell properties are measured as in a conventional flow cytometer. Because cells are repeatedly measured the single-cell rates of change can be determined. The developed instrumentation was applied to measure the variability of ABC transporter activity in a population of human cancer cells and the temperature dependence of constitutively expressed Gfp in yeast. A wide range of transport rates can be observed in the cancer cell population while the single-cell Gfp fluorescence in yeast shows pronounced oscillations in response to temperature shifts. These observations are not detectable at the population level. Therefore, such measurements are useful for investigating cell function as they reveal how variable properties of single cells change over time.

Structure, function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1. 2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes. 3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs. 4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood-brain barrier. 5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins. 6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases. 7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug-drug interactions with altered drug pharmacokinetics and response. 8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.

About a switch: how P-glycoprotein (ABCB1) harnesses the energy of ATP binding and hydrolysis to do mechanical work

The efflux of drugs by the multidrug transporter P-glycoprotein (Pgp; ABCB1) is one of the principal means by which cancer cells evade chemotherapy and exhibit multidrug resistance. Mechanistic studies of Pgp-mediated transport, however, transcend the importance of this protein per se as they help us understand the transport pathway of the ATP-binding cassette proteins in general. The ATP-binding cassette proteins comprise one of the largest protein families, are central to cellular physiology, and constitute important drug targets. The functional unit of Pgp consists of two nucleotide-binding domains (NBD) and two transmembrane domains that are involved in the transport of drug substrates. Early studies postulated that conformational changes as a result of ATP hydrolysis were transmitted to the transmembrane domains bringing about drug transport. More recent structural and biochemical studies on the other hand suggested that ATP binds at the interface of the two NBDs and induces the formation of a closed dimer, and it has been hypothesized that this dimerization and subsequent ATP hydrolysis powers transport. Based on the mutational and biochemical work on Pgp and structural studies with isolated NBDs, we review proposed schemes for the catalytic cycle of ATP hydrolysis and the transport pathway.

Complete Inhibition of P-glycoprotein by Simultaneous Treatment with a Distinct Class of Modulators and the UIC2 Monoclonal Antibody

P-glycoprotein (Pgp) is one of the active efflux pumps that are able to extrude a large variety of chemotherapeutic drugs from the cells, causing multidrug resistance. The conformation-sensitive UIC2 monoclonal antibody potentially inhibits Pgp-mediated substrate transport. However, this inhibition is usually partial, and its extent is variable because UIC2 binds only to 10 to 40% Pgp present in the cell membrane. The rest of the Pgp molecules become recognized by this antibody only in the presence of certain substrates or modulators, including vinblastine, cyclosporine A (CsA), and SDZ PSC 833 (valspodar). Simultaneous application of any of these modulators and UIC2, followed by the removal of the modulator, results in a completely restored steady-state accumulation of various Pgp substrates (calcein-AM, daunorubicin, and 99mTc-hexakis-2-methoxybutylisonitrile), indicating near 100% inhibition of pump activity. Remarkably, the inhibitory binding of the antibody is brought about by coincubation with concentrations of CsA or SDZ PSC 833 approximately 20 times lower than what is necessary for Pgp inhibition when the modulators are applied alone. The feasibility of such a combinative treatment for in vivo multidrug resistance reversal was substantiated by the dramatic increase of daunorubicin accumulation in xenotransplanted Pgp+ tumors in response to a combined treatment with UIC2 and CsA, both administered at doses ineffective when applied alone. These observations establish the combined application of a class of modulators used at low concentrations and of the UIC2 antibody as a novel, specific, and effective way of blocking Pgp function in vivo.

New horizon of MDR1 (P-glycoprotein) study

MDR1 (once P-glycoprotein, now referred to as ABCB1) plays a role as a blood-brain barrier, preventing drug absorption into the brain, and is known to confer multiple drug resistance in cancer chemotherapy. MDR1 is composed of two repeated fragments, and there are six transmembrane domains (TMD) on the N-terminal of each repeat and a nucleotide (ATP) binding domain (NBD) on the C-terminal. These two repeats are dependent but cooperate as one functional molecule, with one pocket for excreting drugs. The 12 TM domains form a funnel facing the outside of cells, and NBD is in cytosol as a dimer. One NBD is composed of the Walker A, Q-loop, ABC-signature and the Walker B for phosphate binding of nucleotide. This tertiary structure of MDR1 is suggested from the structure of the NBD of histidine permease (HisP), clarified by x-ray crystallography. On the model of HisP, the NBD positions described above make a functional domain, and the same NBD structure is found on many other ABC transporters. An experiment with MDR1 gene knockout mice showed the high plasma AUC of drugs in mdr null mice [mdr1a(-/-)] and a high level in the brain, indicating that MDR1 has an efflux function (prevention of absorption) in the intestinal lumen and acts as a barrier of drug uptake in the brain, as well as has the function of urinary and biliary excretion of drugs. The transcription of MDR1 is dependent on two sites; the promoter site (-105/-100)(-245/-141) and the enhancer site (-7864/-7817). Autoantibody from autoimmune hepatitis patients weakly reacted with the extracellular peptide (aa314-aa328 between TM5 and 6) of MDR1 on the outside of the cell membrane, and did not react with peptides in the NBD and in the membrane-spanning region in TM5. There is an ambiguity about the function of MDR1 as GlcCer translocase.

In vivo saturation of the transport of vinblastine and colchicine by P-glycoprotein at the rat blood-brain barrier

PURPOSE

To determine concentration-dependent P-gp-mediated efflux across the luminal membrane of endothelial cells at the blood-brain barrier (BBB) in rats.

METHODS

The transport of radiolabeled colchicine and vinblastine across the rat BBB was measured with or without PSC833, a well known P-gp inhibitor, and within a wide range of colchicine and vinblastine concentration by an in situ brain perfusion. Thus, the difference of brain transport achieved with or without PSC833 gives the P-gp-mediated efflux component of the compound transported through the rat BBB. Cerebral vascular volume was determined by coperfusion with labeled sucrose in all experiments.

RESULTS

Sucrose perfusion indicated that the vascular space was close to normal in all the studies, indicating that the BBB remained intact. P-gp limited the uptake of both colchicine and vinblastine, but the compounds differ in that vinblastine inhibited its own transport. Vinblastine transport was well fitted by a Hill equation giving IC50 at approximately 71 microM, a Hill coefficient (n) approximately 2, and a maximal efflux velocity Jmax of approximately 9 pmol s(-1) g(-1) of brain.

CONCLUSIONS

P-gp at the rat BBB may carry out both capacity-limited and capacity-unlimited transport, depending on the substrate, with pharmacotoxicologic significance for drug brain disposition and risk of drug-drug interactions.

P-GLYCOPROTEIN ACTIVITY IS DECREASED IN CD4+ BUT NOT CD8+ LUNG ALLOGRAFT-INFILTRATING T CELLS DURING ACUTE CELLULAR REJECTION

BACKGROUND

Modulation of P-glycoprotein (P-gp) activity in graft-infiltrating T cells may alter their susceptibility to immunosuppression.

METHODS

P-gp activity was measured by rhodamine efflux in T-cell subsets from bronchoalveolar lavage (BAL) of five healthy volunteers and 27 lung allograft recipients. The effect of T-cell activation on P-gp activity was modeled by stimulation of peripheral blood mononuclear cells with staphylococcal enterotoxin B.

RESULTS

Most BAL T cells expressed memory-effector markers. Patients had a lower proportion of CD4 T cells (P = 0.005), whereas control subjects had CD4-to-CD8 ratios similar to peripheral blood. In controls, basal P-gp activity was greatly increased in both CD4 (35% P-gp active) and CD8 (63%) lung T cells compared with peripheral T cells. Basal P-gp activity was elevated in patient BAL T cells but was lower than control BAL activity (CD4, P = 0.07; CD8, P = 0.03). Lung T cells from transplant patients had modest (CD4) or marked (CD8) increases in substrate-induced P-gp activity compared with normal lung, indicating that P-gp was not irreversibly inhibited. Patients with acute cellular rejection (ACR) had reduced P-gp activity in CD4, but not CD8, BAL T cells compared with patients without ACR (P = 0.004). To determine the relationship between T-cell activation on P-gp modulation, P-gp activity was measured in staphylococcal enterotoxin B-stimulated peripheral blood mononuclear cells. P-gp activity was abrogated in CD71 cycling cells but remained high in a persistent but minor population of resting naive T cells.

CONCLUSIONS

Lung T cells have increased in vivo P-gp activity and therefore may eliminate substrate drugs, resulting in local resistance to immunosuppressive therapy. However, P-gp function is reduced during T-cell activation, providing a window of susceptibility to treatment during ACR.

Analysis of P-glycoprotein-mediated membrane transport in human peripheral blood lymphocytes using the UIC2 shift assay

BACKGROUND

During transport-associated adenosine triphosphate hydrolysis, P-glycoprotein (Pgp) undergoes conformation transitions detected by UIC2, a functional anti-Pgp monoclonal antibody. A newly developed UIC2 shift assay is based on increased UIC2 reactivity in the presence of Pgp substrates. All peripheral blood leukocytes express low Pgp levels. The existing antibody-based detection methods are limited in their sensitivity and require additional techniques to simultaneously analyze Pgp expression and efflux, making it difficult to ascertain the physiologic role of Pgp-mediated transport.

METHODS

We validated the UIC2 shift assay against UIC2 immunostaining and DiOC(2) efflux. The UIC2 shift assay was then used to characterize Pgp functional expression and its physiologic substrates in peripheral blood leukocytes.

RESULTS

A strong correlation was observed between the UIC2 shift assay versus immunostaining and dye efflux tests. The UIC2 shift assay showed improved sensitivity (compared with conventional UIC2 staining) and allowed for simultaneous detection of Pgp expression and function. Using this assay, we identified several new Pgp substrates, including monensin and retinol, and confirmed that interleukin-2 and interferon-gamma can be transported by Pgp.

CONCLUSIONS

Our findings validate the use of the UIC2 shift assay in MDR1 detection and support the idea that Pgp plays a physiologic role in immunoregulation.

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.

Effects of the membrane dipole potential on the interaction of saquinavir with phospholipid membranes and plasma membrane receptors of Caco-2 cells

The combined use of the membrane surface potential fluorescent sensor fluorescein phosphatidylethanolamine (FPE) and the membrane dipole potential fluorescent sensor di-8-ANEPPS to characterize the interaction of molecules with model and cellular membranes and to asses the influence of the dipole potential on the interaction is reported. The study of the human immunodeficiency virus protease inhibitor saquinavir with Caco-2 cells and phospholipid membranes reveals that the compound interacts with the lipidic bilayer of model membranes with a simple hyperbolic binding profile but with Caco-2 cells in a cooperative way involving membrane receptors. Additional studies indicated that colchicine acts as a competitor ligand to saquinavir and suggests, in agreement with other reports, that the identity of the saquinavir "receptor" could be P-glycoprotein or the multiple drug resistance-associated protein. The modification of the magnitude of the membrane dipole potential using compounds such as cholesterol, phloretin, and 6-ketocholestanol influences the binding capacity of saquinavir. Furthermore, removal of cholesterol from the cell membrane using methyl-beta-cyclodextrin significantly decreases the binding capacity of saquinavir. Because removal of cholesterol from the cell membrane has been reported to disrupt membrane domains known as "rafts," our observations imply that the membrane dipole potential plays an important role as a modulator of molecule-membrane interactions in these membrane structures. Such a role is suggested to contribute to the altered behavior of receptor-mediated signaling systems in membrane rafts.