A single membrane-embedded negative charge is critical for recognizing positively charged drugs by the Escherichia coli multidrug resistance protein MdfA.
The nature of the broad substrate specificity phenomenon, as manifested by multidrug resistance proteins, is not yet understood. In the Escherichia coli multidrug transporter, MdfA, the hydrophobicity profile and PhoA fusion analysis have so far identified only one membrane-embedded charged amino acid residue (E26). In order to determine whether this negatively charged residue may play a role in multidrug recognition, we evaluated the expression and function of MdfA constructs mutated at this position. Replacing E26 with the positively charged residue lysine abolished the multidrug resistance activity against positively charged drugs, but retained chloramphenicol efflux and resistance. In contrast, when the negative charge was preserved in a mutant with aspartate instead of E26, chloramphenicol recognition and transport were drastically inhibited; however, the mutant exhibited almost wild-type multidrug resistance activity against lipophilic cations. These results suggest that although the negative charge at position 26 is not essential for active transport, it dictates the multidrug resistance character of MdfA. We show that such a negative charge is also found in other drug resistance transporters, and its possible significance regarding multidrug resistance is discussed. (+info)
Overexpression of the multidrug resistance-associated protein (MRP1) in human heavy metal-selected tumor cells.
Cellular and molecular mechanisms involved in the resistance to cytotoxic heavy metals remain largely to be characterized in mammalian cells. To this end, we have analyzed a metal-resistant variant of the human lung cancer GLC4 cell line that we have selected by a step-wise procedure in potassium antimony tartrate. Antimony-selected cells, termed GLC4/Sb30 cells, poorly accumulated antimony through an enhanced cellular efflux of metal, thus suggesting up-regulation of a membrane export system in these cells. Indeed, GLC4/Sb30 cells were found to display a functional overexpression of the multidrug resistance-associated protein MRP1, a drug export pump, as demonstrated by Western blotting, reverse transcriptase-polymerase chain reaction and calcein accumulation assays. Moreover, MK571, a potent inhibitor of MRP1 activity, was found to markedly down-modulate resistance of GLC4/Sb30 cells to antimony and to decrease cellular export of the metal. Taken together, our data support the conclusion that overexpression of functional MRP1 likely represents one major mechanism by which human cells can escape the cytotoxic effects of heavy metals. (+info)
Influence of tangeretin on tamoxifen's therapeutic benefit in mammary cancer.
BACKGROUND: Tamoxifen and the citrus flavonoid tangeretin exhibit similar inhibitory effects on the growth and invasive properties of human mammary cancer cells in vitro; furthermore, the two agents have displayed additive effects in vitro. In this study, we examined whether tangeretin would enhance tamoxifen's therapeutic benefit in vivo. METHODS: Female nude mice (n = 80) were inoculated subcutaneously with human MCF-7/6 mammary adenocarcinoma cells. Groups of 20 mice were treated orally by adding the following substances to their drinking water: tamoxifen (3 x 10(-5) M), tangeretin (1 x 10(-4) M), tamoxifen plus tangeretin (3 x 10(-5) M plus 1 x 10(-4) M), or solvent. RESULTS AND CONCLUSIONS: Oral treatment of mice with tamoxifen resulted in a statistically significant inhibition of tumor growth compared with solvent treatment (two-sided P = .001). Treatment with tangeretin did not inhibit tumor growth, and addition of this compound to drinking water with tamoxifen completely neutralized tamoxifen's inhibitory effect. The median survival time of tumor-bearing mice treated with tamoxifen plus tangeretin was reduced in comparison with that of mice treated with tamoxifen alone (14 versus 56 weeks; two-sided P = .002). Tangeretin (1 x 10(-6) M or higher) inhibited the cytolytic effect of murine natural killer cells on MCF-7/6 cells in vitro, which may explain why tamoxifen-induced inhibition of tumor growth in mice is abolished when tangeretin is present in drinking water. IMPLICATIONS: We describe an in vivo model to study potential interference of dietary compounds, such as flavonoids, with tamoxifen, which could lead to reduced efficacy of adjuvant therapy. In our study, the tumor growth-inhibiting effect of oral tamoxifen was reversed upon addition of tangeretin to the diet. Our data argue against excessive consumption of tangeretin-added products and supplements by patients with mammary cancer during tamoxifen treatment. (+info)
Class I P-glycoproteins (Pgp) confer multidrug resistance in tumors, but the physiologic function of Pgp in normal tissues remains uncertain. In cells derived from tissues that normally express Pgp, recent data suggest a possible role for Pgp in cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. We investigated the esterification of plasma membrane cholesterol under basal conditions and in response to sphingomyelinase treatment in transfected and drug-selected cell lines expressing differing amounts of functional class I Pgp. Compared with parental NIH 3T3 fibroblasts, cells transfected with human multidrug resistance (MDR1) Pgp esterified more cholesterol both without and with sphingomyelinase. Esterification also was greater in drug-selected Dox 6 myeloma cells than parental 8226 cells, which express low and non-immunodetectable amounts of Pgp, respectively. However, no differences in total plasma membrane cholesterol were detected. Transfection of fibroblasts with the multidrug resistance-associated protein (MRP) did not alter esterification, showing that cholesterol trafficking was not generally affected by ATP-binding cassette transporters. Steroidal (progesterone, dehydroepiandrosterone) and non-steroidal antagonists (verapamil, PSC 833, LY335979, and GF120918) were evaluated for effects on both cholesterol trafficking and the net content of 99mTc-Sestamibi, a reporter of drug transport activity mediated by Pgp. In Pgp-expressing cells treated with nonselective and selective inhibitors, both the kinetics and efficacy of inhibition of cholesterol esterification differed from the antagonism of drug transport mediated by Pgp. Thus, although the data show that greater expression of class I Pgp within a given cell type is associated with enhanced esterification of plasma membrane cholesterol in support of a physiologic function for Pgp in facilitating cholesterol trafficking, the molecular mechanism is dissociated from the conventional drug transport activity of Pgp. (+info)
GR-891: a novel 5-fluorouracil acyclonucleoside prodrug for differentiation therapy in rhabdomyosarcoma cells.
Differentiation therapy provides an alternative treatment of cancer that overcomes the undesirable effects of classical chemotherapy, i.e. cytotoxicity and resistance to drugs. This new approach to cancer therapy focuses on the development of specific agents designed to selectively engage the process of terminal differentiation, leading to the elimination of tumorigenic cells and recovery of normal cell homeostasis. A series of new anti-cancer pyrimidine acyclonucleoside-like compounds were designed and synthesized by structural modifications of 5-fluorouracil, a drug which causes considerable cell toxicity and morbidity, and we evaluated their applicability for differentiation therapy in human rhabdomyosarcoma cells. We tested the pyrimidine derivative GR-891, (RS)-1-[[3-(2-hydroxyethoxy)-1-isopropoxy]propyl]-5-fluorouracil, an active drug which shows low toxicity in vivo and releases acrolein which is an aldehyde with anti-tumour activity. Both GR-891 and 5-fluorouracil caused time- and dose-dependent growth inhibition in vitro; however, GR-891 showed no cytotoxicity at low doses (22.5 micromol l(-1) and 45 micromol l(-1)) and induced terminal myogenic differentiation in RD cells (a rhabdomyosarcoma cell line) treated for 6 days. Changes in morphological features and in protein organization indicated re-entry in the pathway of muscular maturation. Moreover, GR-891 increased adhesion capability mediated by the expression of fibronectin, and did not induce overexpression of P-glycoprotein, the mdr1 gene product, implicated in multidrug resistance. New acyclonucleoside-like compounds such as GR-891 have important potential advantages over 5-fluorouracil because of their lower toxicity and their ability to induce myogenic differentiation in rhabdomyosarcoma cells. Our results suggest that this drug may be useful for differentiation therapy in this type of tumour. (+info)
Induction of MDR1 gene expression by anthracycline analogues in a human drug resistant leukaemia cell line.
The effects of 4-demethoxydaunorubicin (idarubicin, IDA) and MX2, a new morpholino-anthracycline, on up-regulation of the MDR1 gene in the low-level multidrug resistant (MDR) cell line CEM/A7R were compared at similar concentrations (IC10, IC50 and IC90) over a short time exposure (4 and 24 h). The chemosensitivity of each drug was determined by a 3-day cell growth inhibition assay. Compared with epirubicin (EPI), IDA and MX2 were 17- and eightfold more effective in the CEM/A7R line respectively. No cross-resistance to 5-FU was seen in the CEM/A7R line. Verapamil (5 microM) and PSC 833 (1 microM), which dramatically reversed resistance to EPI in the CEM/A7R line, had no sensitizing effect on the resistance of this line to MX2, but slightly decreased resistance to IDA. The sensitivity to 5-FU was unchanged by these modulators. The induction of MDR1 mRNA expression by IDA, MX2 and 5-FU was analysed by Northern blotting and semiquantitatively assessed by scanning Northern blots on a phosphorimager. The relative level of MDR1 expression was expressed as a ratio of MDR1 mRNA to the internal RNA control glyceraldehyde-3-phosphate dehydrogenase (GAPDH). IDA, MX2 and 5-FU differentially up-regulated MDR1 mRNA in the CEM/A7R line in a dose-dependent manner. Both IDA and MX2 induced MDR1 expression within 4 h. 5-FU up-regulated MDR1 expression only when drug exposure was prolonged to 24 h. Based on MRK 16 binding, flow cytometric analysis of P-glycoprotein (Pgp) expression paralleled the increase in MDR1 mRNA levels. For the three anthracyclines, the increase in MDR1 expression was stable in cells grown in the absence of drug for more than 3 weeks after drug treatment. The induction of MDR1 expression by 5-FU was transient, associated with a rapid decrease in the increased Pgp levels which returned to baseline 72 h after the removal of 5-FU. This study demonstrates that MDR1 expression can be induced by analogues of anthracyclines not pumped by Pgp, and that this induction appears to be stable despite a 3-week drug-free period. (+info)
Transport of rhodamine 123, a P-glycoprotein substrate, across rat intestine and Caco-2 cell monolayers in the presence of cytochrome P-450 3A-related compounds.
Effects of cytochrome P-450 3A- and P-glycoprotein (P-gp)-related compounds, erythromycin, midazolam, ketoconazole, verapamil, and quinidine, on transport of rhodamine 123 (Rho-123), a P-gp substrate, were studied in rat intestine and in Caco-2 cells. Ileum was mainly used in rat studies because this segment showed greater P-gp-mediated Rho-123 transport. In an in vitro everted rat ileum, all the compounds examined significantly inhibited the transport of Rho-123 from serosal to mucosal surfaces across the intestine, with different inhibitory potencies among these compounds. In an in vivo rat study, the exsorption of Rho-123 from blood to the intestinal lumen, which was evaluated as exsorption clearance of Rho-123 under a steady-state plasma concentration of Rho-123, was also inhibited when these compounds were added to the intestinal lumen. Similarly, transepithelial transport of Rho-123 from the basolateral to apical side across Caco-2 cell monolayers was inhibited by these compounds. A linear relationship was observed in their inhibitory potencies on Rho-123 transport between in vitro and in vivo studies using rat ileum and between studies with rat ileum and Caco-2 cells. P-gp-mediated transport across the intestine was found to be inhibited not only by P-gp-related but also by all the cytochrome P-450 3A-related compounds examined. Within experimental error, the relative inhibitory potencies were the same between the studies with rat ileum (in vivo, in vitro) and those with Caco-2 cells. Thus, it is suggested that the function of P-gp and its sensitivity to these drugs may be similar in rat intestine and Caco-2 cells. (+info)
Stimulation of P-glycoprotein-mediated drug transport by prazosin and progesterone. Evidence for a third drug-binding site.
P-glycoprotein is a plasma membrane protein of mammalian cells that confers multidrug resistance by acting as a broad-specificity, ATP-dependent efflux transporter of diverse lipophilic neutral or cationic compounds. Previously, we identified two positively cooperative drug-binding sites of P-glycoprotein involved in transport [Shapiro, A. B. & Ling, V. (1997) Eur. J. Biochem. 250, 130-137]. The H site is selective for Hoechst 33342 and colchicine. The R site is selective for rhodamine 123 and anthracyclines. Substrate binding to one site stimulates transport by the other. In this paper, we show that prazosin and progesterone stimulate the transport of both Hoechst 33342 and rhodamine 123. Rhodamine 123 and prazosin (or progesterone) in combination stimulate Hoechst 33342 transport in an additive manner. In contrast, Hoechst 33342 and either prazosin or progesterone interfere with each other, so that the stimulatory effect of the combination on rhodamine 123 transport is less than that of each individually. Non-P-glycoprotein-specific effects of prazosin on membrane fluidity and permeability were excluded. These results indicate the existence of a third drug-binding site on P-glycoprotein with a positive allosteric effect on drug transport by the H and R sites. This allosteric site appears to be one of the sites of photoaffinity labeling of P-glycoprotein by [125I]iodoarylazidoprazosin [Safa, A. R., Agresti, M., Bryk, D. & Tamai, I. (1994) Biochemistry 33, 256-265] and is likely not to be capable of drug transport. (+info)