Improved methotrexate therapy of murine tumors obtained by probenecid-mediated pharmacological modulation at the level of membrane transport.
Explore the source record for details and available documents.
Animals↗
SEARCH · PubMed Health
Results for “Membrane Transport Modulators”
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Sulfhydryl group involvement in the modulation of neutral amino acid transport in thymocyte membrane vesicles.
Membrane vesicles from rat thymocytes accumulate 2-aminoisobutyric acid in the presence of 0. 1 M NaCl. Uptake is half maximal between 15 and 30 seconds after addition of the amino acid and reaches a plateau value after about 2 minutes. The uptake of 2-aminoisobutyric acid can be modulated by various sulfhydryl reagents. Reduced glutathione leads to an inhibition of uptake whereas oxidized glutathione increases uptake. Agents such as insulin and diamide which can induce disulfide formation lead to an activation of transport. These date indicate that uptake of the Na+-dependent amino acid, 2-aminoisobutyric acid, in thymocytes is modulated by a putative plasma membrane, sulfhydryl-containing protein.
Amino Acids↗
Effects of nonionic surfactants on membrane transporters in Caco-2 cell monolayers.
The objectives of this study were (1) to investigate the transporter inhibition activity of three nonionic surfactants on P-glycoprotein, the human intestinal peptide transporter, and the monocarboxylic acid transporter in Caco-2 cell monolayers, and (2) to evaluate the role of membrane fluidity and protein kinase C in surfactant-induced transporter inhibition. All three surfactants inhibited P-glycoprotein (P-gp). Over a range from 0 to 1 mM, Tween 80 and Cremophor EL increased apical-to-basolateral permeability (AP-BL) and decreased basolateral-to-apical (BL-AP) permeability of the P-gp substrate rhodamine 123. Vitamin E TPGS's effect was equally large, but essentially only reduced the BL-AP permeability of rhodamine 123, and did so at a vitamin E TPGS concentration of only 0.025 mM. These P-gp inhibition effects would appear to be related to these excipients' modulation of membrane fluidity, where Tween 80 and Cremophor EL fluidized cell lipid bilayers, while vitamin E TPGS rigidized lipid bilayers. However, among the three surfactants, only Tween 80 inhibited the peptide transporter, as measured by glycyl sarcosine permeability. Likewise, only Cremophor EL inhibited the monocarboxylic acid transporter, as measured by benzoic acid permeability. Nevertheless, at least one of these three surfactants inhibited each P-gp, the human intestinal peptide transporter, and the monocarboxylic acid transporter. A common functional feature of these three surfactants was their ability to modulate fluidity, although results indicate that even strong membrane fluidity modulation alone was not sufficient to reduce transporter activity. N-octyl glucoside, a nonionic surfactant that did not modulate membrane fluidity, did not affect transporter functioning. Protein kinase C inhibitors failed to affect rhodamine 123 and glycyl sarcosine permeability, suggesting protein kinase C inhibition was not the mechanism of transporter inhibition. These results suggest that surfactants can inhibit multiple transporters but that changes in membrane fluidity may not be a generalized mechanism to reduce transporter activity.
ATP Binding Cassette Transporter, Subfamily B, Mem↗
Activities of novel nonglycosidic epipodophyllotoxins in etoposide-sensitive and -resistant variants of human KB cells, P-388 cells, and in vivo multidrug-resistant murine leukemia cells.
Previous structure-activity studies of the antitumor compound etoposide (VP-16) have suggested that replacement of the glycoside moiety could afford therapeutically active analogues with different biochemical determinants for cellular accumulation and drug resistance. In the present report, 10 analogues of VP-16 in which the glycosidyl moiety was replaced with alkyl or arylamino substituents exhibited 5-10-fold better binding affinity for topoisomerase II/DNA complex in human KB cells. A similar increase in the binding affinity was observed in an isolated-nuclei model. The analogues displayed greater or comparable potency to VP-16 in cell growth-inhibition studies and were less affected by cell membrane-associated drug resistance mechanisms, as exemplified by overexpressions of P-glycoprotein multidrug-resistance gene or multidrug resistance-associated protein. Interestingly, in animal studies, analogues least affected by the membrane transport-deficiency phenotypes exhibited low therapeutic index values, thus suggesting that highly efficient modulation of cellular membrane transport defects could perturb the selectivity of antitumor agents for cancer cells. This report also suggests a new method of quantifying drug-induced protein-linked DNA breaks by graphically determining the apparent dissociation-inhibition constant (Kdi) for the inhibitors.
ATP Binding Cassette Transporter, Subfamily B, Mem↗
Anion transport activity in the human erythrocyte membrane modulated by proteolytic digestion of the 38,000-dalton fragment in Band 3.
Extracellular treatment of human erythrocytes with papain completely converted the chymotryptic 38,000-dalton fragment of Band 3 to the 29,000-dalton fragment and inhibited the transport of inorganic phosphate in the cells. The inhibition, however, was not complete, indicating the presence of two components in the anion-transport system: the one resistant to papain digestion and the other sensitive to the digestion. The latter activity is well correlated with the degradation of the 38,000-dalton fragment. The activity remaining in the cells treated with papain was markedly different from that of the control cells. The remaining activity was not inhibited by pyridoxal phosphate and dinitrostilbene-2,2'-disulfonic acid, potent inhibitors to the anion transport, whereas phenyl phosphate inhibited the activities of both papain-treated and control cells. The results indicate that the anion-transport system consists of multiple anion-binding sites and a part of the system which is sensitive to pyridoxal phosphate and dinitrostilbene-2,2'-disulfonic acid was located in the papain-sensitive portion of 38,000-dalton fragment. A possible model of the anion-transport system was presented.
4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
Cholesterol modulates rat renal brush border membrane phosphate transport.
In dietary phosphate (Pi) deprivation and in aging there is an inverse correlation between renal proximal tubular brush border membrane (BBM) cholesterol (Chol) content, BBM fluidity, and BBM sodium gradient-dependent Pi transport activity (Na-Pi cotransport). The purpose of this study was to determine whether in vitro enrichment of renal BBM with Chol has a direct modulating effect on Na-Pi cotransport. 12 and 24 mol % increases in Chol content caused dose-dependent decreases in Na-Pi cotransport activity, 2,000 in control, vs. 1,450 in Chol (+12%), vs. 900 pmol/5 s/mg BBM protein in Chol (+24%), all P less than 0.01, which was paralleled by dose-dependent increases in the fluorescence anisotropy of diphenylhexatriene, rDPH, i.e., decrease in BBM fluidity, 0.203 in control, vs. 0.210 in Chol (+12%), vs. 0.219 in Chol (+24%), all P less than 0.01. We found that increasing ambient temperature, which increases BBM fluidity independent of changes in Chol content, increased Na-Pi cotransport. When Na-Pi cotransport was analyzed as a function of BBM fluidity, 1/rDPH, we found that at an equivalent BBM fluidity BBM Chol enrichment still resulted in a dose-dependent decrease in Na-Pi cotransport. Finally, in BBM isolated from rats fed a low Pi diet in vitro enrichment with Chol completely reversed the adaptive increases in Na-Pi cotransport and fluidity. Our study therefore, indicates that Chol is a direct modulator of renal BBM Na-Pi cotransport activity, and that in vivo alterations in BBM Chol content most likely plays an important role in the regulation of renal tubular Pi transport.
Alkaline Phosphatase↗
Apoptotic and antiapoptotic mechanisms in stroke.
Apoptosis is a form of programmed cell death that occurs in neurons during development of the nervous system and may also be a prominent form of neuronal death in chronic neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Recent findings also implicate apoptosis in neuronal degeneration after ischemic brain injury in animal models of stroke. Activation of both apoptotic and antiapoptotic signaling cascades occurs in neurons in animal and cell culture models of stroke. Apoptotic cascades involve: increased levels of intracellular oxyradicals and calcium; induction of expression of proteins such as Par-4 (prostate apoptosis response-4), which act by promoting mitochondrial dysfunction and suppressing antiapoptotic mechanisms; mitochondrial membrane depolarization, calcium uptake, and release of factors (e.g., cytochrome c) that ultimately induce nuclear DNA condensation and fragmentation; activation of cysteine proteases of the caspase family; activation of transcription factors such as AP-1 that may induce expression of "killer genes." Antiapoptotic signaling pathways are activated by neurotrophic factors, certain cytokines, and increases in oxidative and metabolic stress. Such protective pathways include: activation of the transcription factors (e.g., nuclear factor-kappa B, NF-kappa B) that induce expression of stress proteins, antioxidant enzymes, and calcium-regulating proteins; phosphorylation-mediated modulation of ion channels and membrane transporters; cytoskeletal alterations that modulate calcium homeostasis; and modulation of proteins that stabilize mitochondrial function (e.g., Bcl-2). Intervention studies in experimental stroke models have identified a battery of approaches of potential benefit in reducing neuronal death in stroke patients, including administration of antioxidants, calcium-stabilizing agents, caspase inhibitors, and agents that activate NF-kappa B. Interestingly, recent studies suggest novel dietary approaches (e.g., food restriction and supplementation with antioxidants) that may reduce brain damage following stroke.
Animals↗
Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone.
Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.
Amiloride↗
Stearoyl-CoA desaturase inhibits ATP-binding cassette transporter A1-mediated cholesterol efflux and modulates membrane domain structure.
Liver X receptor/retinoid X receptor (LXR/RXR) transcription factors have been found to induce a number of genes involved in the regulation of cellular cholesterol efflux, including the ATP-binding cassette transporter A1 (ABCA1), which mediates the active efflux of cellular cholesterol and phospholipids to extracellular acceptors, such as apolipoprotein A-I (apoA-I). In a screen for macrophage LXR/RXR target genes, we identified stearoyl-CoA desaturases 1 and 2 (Scd1 and Scd2), and subsequently tested the hypothesis that SCD activity might modulate cellular cholesterol efflux. In HEK 293 cells co-transfection of ABCA1 with either SCD1 or SCD2 inhibited ABCA1-mediated cholesterol efflux but not phospholipid efflux. In Chinese hamster ovary (CHO) cells with moderate stable overexpression of SCD1, cholesterol efflux to apoA-I was inhibited by 73%, whereas phospholipid efflux and ABCA1 protein levels were unchanged. In contrast, cholesterol efflux to HDL(2), which is not dependent on ABCA1, was increased 2-fold in CHO-SCD1 cells. The effect of SCD on cholesterol efflux to apoA-I was independent of acyl-CoA:cholesterol acyltransferase (ACAT) activity. SCD activity led to an increased content of plasma membrane monounsaturated fatty acids (18:1) at the expense of saturated fatty acids (18:0). As shown by confocal microscopy, SCD overexpression led to a decrease of Triton X-100-resistant domains in the plasma membrane, indicating a decrease in membrane-ordered regions. The data suggest that SCD changes membrane organization and depletes a specific pool of membrane cholesterol supporting ABCA1-mediated efflux, whereas increasing availability of cholesterol for passive efflux by HDL(2). ABCA1-mediated cholesterol and phospholipid efflux may be uncoupled in pathological states associated with high SCD activity, as in hyperinsulinemic obese mice, or in animals treated with LXR activators.
ATP-Binding Cassette Transporters↗
Dexamethasone modulates rat renal brush border membrane phosphate transporter mRNA and protein abundance and glycosphingolipid composition.
Glucocorticoids are important regulators of renal phosphate transport. This study investigates the role of alterations in renal brush border membrane (BBM) sodium gradient-dependent phosphate transport (Na-Pi cotransporter) mRNA and protein abundance in the dexamethasone induced inhibition of Na-Pi cotransport in the rat. Dexamethasone administration for 4 d caused a 1.5-fold increase in the Vmax of Na-Pi cotransport (1785 +/- 119 vs. 2759 +/- 375 pmol/5 s per mg BBM protein in control, P < 0.01), which was paralleled by a 2.5-fold decrease in the abundance of Na-Pi mRNA and Na-Pi protein. There was also a 1.7-fold increase in BBM glucosylceramide content (528 +/- 63 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02). To determine whether the alteration in glucosylceramide content per se played a functional role in the decrease in Na-Pi cotransport, control rats were treated with the glucosylceramide synthase inhibitor, D-threo-1-phenyl-2-decanoyl-amino-3-morpholino-1-propanol (PDMP). The resultant 1.5-fold decrease in BBM glucosylceramide content (199 +/- 19 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02) was associated with a 1.4-fold increase in Na-Pi cotransport activity (1422 +/- 73 vs. 1048 +/- 85 pmol/5 s per mg BBM protein in control, P < 0.01), and a 1.5-fold increase in BBM Na-Pi protein abundance. Thus, dexamethasone-induced inhibition of Na-Pi cotransport is associated with a decrease in BBM Na-Pi cotransporter abundance, and an increase in glucosylceramide. Since primary alteration in BBM glucosylceramide content per se directly and selectively modulates BBM Na-Pi cotransport activity and Na-Pi protein abundance, we propose that the increase in BBM glucosylceramide content plays an important role in mediating the inhibitory effect of dexamethasone on Na-Pi cotransport activity.
Animals↗
On the presence of met 5-enkephalin receptors on the plasma membrane of Deiters' neurons and their modulation of GABA transport.
Met 5-enkephalin blocks the active transport of GABA across plasma membranes of rabbit Deiters' neurons prepared by microdissection. Such an effect is receptor-mediated, being reversed by the antagonist Naloxone. This result shows that Deiters' neurons bear enkephalin receptors, and suggests a hypothesis on the mechanism of enkephalin-mediated neuronal inhibition.
Animals↗
Ca(2+)-dependent protein kinases modulate proline transport across the renal brush-border membrane.
The cellular mechanisms controlling reabsorption of amino acids in the renal proximal tubule are unknown. Ca(2+)-dependent protein kinases modulate the activity of several ion channels and carriers in the kidney. The role of these enzymes in regulating tubular amino acid transport has not been established. We investigated the effect of Ca(2+)- and phospholipid-dependent protein kinase C (PKC) and Ca2+/calmodulin-dependent protein kinase II (CaMK II) on Na(+)- and Cl(-)-dependent proline transport across the rat renal brush-border membrane (BBM). Bioassays utilizing selective peptide substrates for Ca(2+)-dependent protein kinases demonstrated the presence of PKC and CaMK II in the BBM. Renal brush-border membrane vesicles (BBMV) were phosphorylated using the "hyposmotic shock" technique. Endogenous (membrane-bound) CaMK II and PKC, as well as exogenous, highly purified PKC inhibited NaCl-linked proline uptake by phosphorylated, lysed/resealed BBMV compared with control vesicles. The inhibitory effect of Ca2+ on proline transport, without the presence of other kinase activators, was mediated by activation of endogenous CaMK II. The CaMK II- and PKC-induced inhibition of proline uptake was reversed by the specific kinase inhibitor peptides CaMK II-(281-302) and PKC-(19-31), respectively. These data suggest that Ca(2+)-dependent protein kinase-mediated phosphorylation inhibits NaCl-dependent proline transport across the tubular luminal membrane.
Animals↗
Modulation of red cell K+ transport by membrane lipids.
Explore the source record for details and available documents.
Biological Transport↗
Modulation of glycosylation and transport of viral membrane glycoproteins by a sodium ionophore.
Analysis of viral glycoprotein expression on surfaces of monensin-treated cells using a fluorescence-activated cell sorter (FACS) demonstrated that the sodium ionophore completely inhibited the appearance of the vesicular stomatitis virus (VSV) G protein on (Madin-Darby canine kidney) MDCK cell surfaces. In contrast, the expression of the influenza virus hemagglutinin (HA) glycoprotein on the surfaces of MDCK cells was observed to occur at high levels, and the time course of its appearance was not altered by the ionophore. Viral protein synthesis was not inhibited by monensin in either VSV- or influenza virus-infected cells. However, the electrophoretic mobilities of viral glycoproteins were altered, and analysis of pronase-derived glycopeptides by gel filtration indicated that the addition of sialic acid residues to the VSV G protein was impaired in monensin-treated cells. Reduced incorporation of fucose and galactose into influenza virus HA was observed in the presence of the ionophore, but the incompletely processed HA protein was cleaved, transported to the cell surface, and incorporated into budding virus particles. In contrast to the differential effects of monensin on VSV and influenza virus replication previously observed in monolayer cultures of MDCK cells, yields of both viruses were found to be significantly reduced by high concentrations of monensin in suspension cultures, indicating that cellular architecture may play a role in determining the sensitivity of virus replication to the drug. Nigericin, an ionophore that facilitates transport of potassium ions across membranes, blocked the replication of both influenza virus and VSV in MDCK cell monolayers, indicating that the ion specificity of ionophores influences their effect on the replication of enveloped viruses.
Animals↗
Lipid modulation of transport proteins in vertebrate cell membranes.
Explore the source record for details and available documents.
Animals↗
1-Methyl-4-phenylpridinium (MPP+)-induced functional run-down of GABA(A) receptor-mediated currents in acutely dissociated dopaminergic neurons.
We have evaluated GABA(A)receptor function during treatment of 1-methyl-4-phenylpridinium (MPP+) using patch-clamp perforated whole-cell recording techniques in acutely dissociated dopaminergic (DAergic) neurons from rat substantia nigra compacta (SNc). Gamma-aminobutyric acid (GABA), glutamate or glycine induced inward currents (I(GABA), I(Glu), I(Gly)) at a holding potential (VH) of -45 mV. The I(GABA) was reversibly blocked by the GABA(A) receptor antagonist, bicuculline, suggesting that I(GABA) is mediated through the activation of GABA(A) receptors. During extracellular perfusion of MPP+ (1-10 microm), I(GABA) , but neither I(Glu) nor I(Gly), declined (termed run-down) with repetitive agonist applications, indicating that the MPP+-induced I(GABA) run-down occurred earlier than I(Gly) or I(Glu) under our experimental conditions. The MPP+-induced I(GABA) run-down can be prevented by a DA transporter inhibitor, mazindol, and can be mimicked by a metabolic inhibitor, rotenone. Using conventional whole-cell recording with different concentrations of ATP in the pipette solution, I(GABA) run-down can be induced by decreasing intracellular ATP concentrations, or prevented by supplying intracellular ATP, indicating that I(GABA) run-down is dependent on intracellular ATP concentrations. A GABA(A) receptor positive modulator, pentobarbital (PB), potentiated the declined I(GABA) and eliminated I(GABA) run-down. Corresponding to these patch-clamp data, tyrosine hydroxylase (TH) immunohistochemical staining showed that TH-positive cell loss was protected by PB during MPP+ perfusion. It is concluded that extracellular perfusion of MPP+ induces a functional run-down of GABA(A) receptors, which may cause an imbalance of excitation and inhibition of DAergic neurons.
1-Methyl-4-phenylpyridinium↗
Tetracycline resistance in Mycoplasma hominis.
The composition of the cytoplasmic membrane of a clinical isolate of Mycoplasma hominis that was resistant to tetracycline (minimal inhibitory concentration, 30 micrograms/ml) was compared with that of a susceptible strain (minimal inhibitory concentration, less than 1 microgram/ml). Neither differences in passive permeability of the lipid portion of the membrane to the drug nor modulation of active transport by membrane lipids explained the resistance to tetracycline. Polyacrylamide gel electrophoresis revealed several differences between the membrane protein composition of the two strains. One 25,000-dalton protein was prominent in the membrane of the resistant strain but almost nonexistent in that of the sensitive strain. Therefore, the resistance may have been correlated with differences in protein composition. Preliminary evidence suggested that the resistance trait was plasmid-mediated.
Cell Membrane↗
Control of NMDA receptor activation by a glycine transporter co-expressed in Xenopus oocytes.
We present evidence that membrane transporters can control the membrane receptor's agonist concentration in restricted extracellular spaces of a biological model. The model is constructed by co-expressing glycine/Na/Cl cotransporters (GLYT1b) and NMDA receptors (NMDARs) (composed of the subunits NR1 and NR2A or NR2B) in Xenopus oocytes. We use the high-affinity glycine site of the NMDARs as a sensor of the actual juxtamembrane glycine concentration. We show that glycine uptake by GLYT1b dramatically reduces NMDAR currents by reducing the glycine concentration in extracellular spaces in which diffusion is restricted. This effect appears only in oocytes in which GLYT1b and NMDAR are co-expressed. It is Na+- and voltage-dependent, and is abolished when Na+ is replaced by Li+ and when glycine is replaced by D-serine (a coagonist of the NMDAR that is not transported by GLYT1b). These results demonstrate the ability of the GLYT transporter to reduce glycine concentration at the level of NMDARs in restricted diffusion spaces. This observation could account for a prevalent role of membrane transporters in the modulation of synapse transmission in the CNS. From a more general point of view, our results draw attention to possible significant discrepancies between local concentrations at the level of substrate targets in biological membranes and their concentration in the bulk solution when membrane transporters are present.
Amino Acid Transport Systems, Neutral↗