Conversion between two cytochalasin B-binding states of the human GLUT1 glucose transporter

Glucose-Functionalized Liposomes for Reducing False Positives in Cancer Diagnosis

Fluorodeoxyglucose-positron emission tomography (¹⁸F-FDG-PET) is a powerful tool for cancer detection, staging, and follow-up. However, ¹⁸F-FDG-PET imaging has high rates of false positives, as it cannot distinguish between tumor and inflammation regions that both feature increased glucose metabolic activity. In the present study, we engineered liposomes coated with glucose and the chelator dodecane tetraacetic acid (DOTA) complexed with copper, to serve as a diagnostic technology for differentiating between cancer and inflammation. This liposome technology is based on FDA-approved materials and enables complexation with metal cations and radionuclides. We found that these liposomes were preferentially uptaken by cancer cell lines with high metabolic activity, mediated via glucose transporter-1. In vivo, these liposomes were avidly uptaken by tumors, as compared to liposomes without glucose coating. Moreover, in a combined tumor-inflammation mouse model, these liposomes accumulated in the tumor tissue and not in the inflammation region. Thus, this technology shows high specificity for tumors while evading inflammation and has potential for rapid translation to the clinic and integration with existing PET imaging systems, for effective reduction of false positives in cancer diagnosis.

Uptake mechanism of metabolic-targeted gold nanoparticles

AIM

To elucidate the interactions, uptake mechanisms and cytotoxicity profile of glucose-functionalized gold nanoparticles (2GF-GNPs), for expanding and advancing the recently proposed technology of metabolic-based cancer detection to a variety of cancer diseases.

METHODS

Several cell types with different metabolic features were used to assess the involvement of GLUT-1 and different endocytosis pathways in 2GF-GNP uptake, and the cytotoxicity profile of 2GF-GNPs.

RESULTS

Cellular uptake of 2GF-GNP strongly correlated with GLUT-1 surface expression, and occurred mainly through clathrin-mediated endocytosis. 2GF-GNPs showed no toxic effect on cell cycle and proliferation.

CONCLUSION

These findings promote development of metabolic-based cancer detection technologies, and suggest that 2GF-GNPs may enable specific cancer detection in a wide range of tumors characterized by high GLUT-1 expression.

Differentiating Between Cancer and Inflammation: A Metabolic-Based Method for Functional Computed Tomography Imaging

One of the main limitations of the highly used cancer imaging technique, PET-CT, is its inability to distinguish between cancerous lesions and post treatment inflammatory conditions. The reason for this lack of specificity is that [(18)F]FDG-PET is based on increased glucose metabolic activity, which characterizes both cancerous tissues and inflammatory cells. To overcome this limitation, we developed a nanoparticle-based approach, utilizing glucose-functionalized gold nanoparticles (GF-GNPs) as a metabolically targeted CT contrast agent. Our approach demonstrates specific tumor targeting and has successfully distinguished between cancer and inflammatory processes in a combined tumor-inflammation mouse model, due to dissimilarities in angiogenesis occurring under different pathologic conditions. This study provides a set of capabilities in cancer detection, staging and follow-up, and can be applicable to a wide range of cancers that exhibit high metabolic activity.

Drug partition chromatography on immobilized porcine intestinal brush border membranes

We immobilized porcine intestinal brush border membrane vesicles (BBMVs) for chromatographic analyses of drug partitioning into the membranes determined as Ks, the drug retention per phospholipid amount. For positive and neutral drugs Ks decreased day by day, whereas Ks for negative drugs increased marginally. Similar results on vesicle-lipid liposomes indicated a gradual loss of negative charge from the columns. The Ks values for positive drugs were higher than those for negative drugs with the same octanol/water partitioning or the same Ks on egg yolk phospholipid bilayers. Electrostatic interactions seem to be important for the partitioning of charged drugs into brush border membranes.

On the oligomeric state of the red blood cell glucose transporter GLUT1

We stripped human red blood cell membranes of cytoskeleton proteins at pH 12 without reductant, partially solubilized the obtained vesicles by use of octaethylene glycol n-dodecyl ether and purified the glucose transporter GLUT1 by anion-exchange chromatography followed by sulfhydryl-affinity chromatography, which removed most of the nucleoside transporter (NT) and the lipids. Eighty percent of the sulfhydryl-bound GLUT1 could be eluted with sodium dodecyl sulfate (SDS) indicating that the bound protein was multimeric. Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-ToF-MS) of the trypsinized major SDS-PAGE zone of the purified material identified GLUT1 but no other membrane protein. Transmembrane helices 1 and 8 were among the detected fragments. The reconstituted purified GLUT1 showed glucose transport activity, although only approximately 0.05 high-affinity cytochalasin B (CB) binding sites were present per GLUT1 monomer. The vesicles used as starting material for the purification showed 0.4 CB sites per GLUT1 monomer, similar to vesicles prepared in the presence of dithioerythritol. The data are consistent with the coexistence of monomeric GLUT1 with high-affinity CB-binding activity and preferentially solubilized multimeric GLUT1 with no CB-binding activity in the red blood cell membrane vesicles prepared without reductant.

Centrifugal and chromatographic analyses of tryptophan and tyrosine uptake by red blood cells and GLUT1 proteoliposomes with permeability estimates and observations on dihydrocytochalasin B

We analyzed transport into liposomes and proteoliposomes, separated the free and internalized radioactively labeled substrates by size-exclusion chromatography (SEC) and observed a net influx owing to nonfacilitated diffusion across the lipid bilayers during the separation. The permeabilities (10(-9) cm/s) of glucose transporter (GLUT1) proteoliposomes were estimated to be 4.6, 1.0, 1.4 and 2.1 for D-glucose, L-glucose, L-Tyr and L-Trp, respectively; 15, 3.3, 5.1 and 2.1 times higher than the corresponding permeabilities of liposomes. These values indicated that GLUT1 did not transport Tyr or Trp, or transported Tyr, and only Tyr, slowly. This interpretation was supported by further analyses. Dihydrocytochalasin B inhibited the transport of Tyr and, partially, Trp into human red blood cells (centrifugal analyses). It did not inhibit Tyr and Trp influx into GLUT1 proteoliposomes, but partitioned strongly into the bilayers and seemed to make them fragile. The GLUT1 inhibitor cytochalasin B and the GLUT1 substrate 2-deoxy-D-glucose did not inhibit Tyr transport into the cells. Upon immobilized biomembrane affinity chromatography, Trp decreased the cytochalasin B retardation by GLUT1 only at levels far above the physiological Trp concentration. Ethanol (commonly added to aqueous solutions for enhancing a compound's solubility) halved the retardation at 4% (v/v) concentration. Drastic modification of the SEC method is required to allow permeability measurements with nonlabeled and highly permeable substrates.

Improved lectin-mediated immobilization of human red blood cells in superporous agarose beads

A new type of agarose bead, superporous agarose, was used as a gel support for immobilization of human red blood cells (RBCs) mediated by wheat germ lectin. The number of immobilized cells was similar to that obtained with commercial wheat germ lectin-agarose but the cell stability appeared to be superior. This allowed improved frontal affinity chromatographic analyses of cytochalasin B (CB)-binding to the glucose transporter GLUT1 which established a ratio of one CB-binding site per GLUT1 dimer for both plain RBCs or those treated with different poly amino acids. The measured dissociation constants, 70+/-14 nM for CB and 12+/-3 mM for glucose binding to GLUT1, are similar to those reported earlier.

Interactions of ATP, oestradiol, genistein and the anti-oestrogens, faslodex (ICI 182780) and tamoxifen, with the human erythrocyte glucose transporter, GLUT1

17 beta-Oestradiol (ED when subscript to K) and the phytoestrogen isoflavone genistein (GEN) inhibit glucose transport in human erythrocytes and erythrocyte ghosts. The selective oestrogen receptor modulators or anti-oestrogens, faslodex (ICI 182780) (FAS) and tamoxifen (TAM), competitively antagonize oestradiol inhibition of glucose exit from erythrocytes (K(i(ED/FAS))=2.84+/-0.16 microM and K(i(ED/TAM))=100+/-2 nM). Faslodex has no significant inhibitory effect on glucose exit, but tamoxifen alone inhibits glucose exit (K(i(TAM))=300+/-100 nM). In ghosts, ATP (1-4 mM) competitively antagonizes oestradiol, genistein and cytochalasin B (CB)-dependent inhibitions of glucose exit, (K(i(ATP/ED))=2.5+/-0.23 mM, K(i(ATP/GEN))=0.99+/-0.17 mM and K(i(ATP/CB))=0.76+/-0.08 mM). Tamoxifen and faslodex reverse oestradiol-dependent inhibition of glucose exit with ATP>1 mM (K(i(ED/TAM))=130+/-5 nM and K(i(ED/FAS))=2.7+/-0.9 microM). The cytoplasmic surface of the glucose transporter (GLUT)1 contains four sequences with close homologies to sequences in the ligand-binding domain of human oestrogen receptor beta (hesr-2). One homology is adjacent to the Walker ATP-binding motif II (GLUT1, residues 225-229) in the large cytoplasmic segment linking transmembrane helices 6 and 7; another GLUT (residues 421-423) contains the Walker ATP-binding motif III. Mapping of these regions on to a three-dimensional template of GLUT indicates that a possible oestrogen-binding site lies between His(337), Arg(349) and Glu(249) at the cytoplasmic entrance to the hydrophilic pore spanning GLUT, which have a similar topology to His(475), Glu(305) and Arg(346) in hesr-2 that anchor the head and tail hydroxy groups of oestradiol and genistein, and thus are suitably placed to provide an ATP-sensitive oestrogen binding site that could modulate glucose export.

Immobilized-biomembrane affinity chromatography for binding studies of membrane proteins

Analyses of specific interactions between solutes and a membrane protein can serve to characterize the protein. Frontal affinity chromatography of an interactant on a column containing the membrane protein immobilized in a lipid environment is a simple and robust approach for series of experiments with particular protein molecules. Regression analysis of the retention volumes at a series of interactant concentrations shows the affinity of the protein for the interactant and the amount of active binding sites. The higher the affinity, the fewer sites are required to give sufficient retention. Competition experiments provide the affinities of even weakly binding solutes and the non-specific retention of the primary interactant. Hummel and Dreyer size-exclusion chromatography allows complementary analyses of non-immobilized membrane materials. Analyses of the human facilitative glucose transporter GLUT1 by use of the inhibitor cytochalasin B (radioactively labeled) and the competitive substrate D-glucose (non-labeled) showed that GLUT1 interconverted between two states, exhibiting one or two cytochalasin B-binding sites per two GLUTI monomers, dependent on the membrane composition and environment. Similar analyses of a nucleoside transporter, a photosynthetic reaction center, nicotinic acetylcholine receptors and a P-glycoprotein, alternative techniques, and immobilized-liposome chromatographic approaches are presented briefly.

Effects of cholesterol and model transmembrane proteins on drug partitioning into lipid bilayers as analysed by immobilized-liposome chromatography

We have analysed how cholesterol and transmembrane proteins in phospholipid bilayers modulate drug partitioning into the bilayers. For this purpose we determined the chromatographic retention of drugs on liposomes or proteoliposomes entrapped in gel beads. The drug retention per phospholipid amount (the capacity factor Ks) reflects the drug partitioning. Cholesterol in the bilayers decreased the Ks value and hence the partitioning into the membrane in proportion to the cholesterol fraction. On average this cholesterol effect decreased with increasing temperature. Model transmembrane proteins, the glucose transporter GLUT1 and bacteriorhodopsin, interacted electrostatically with charged drugs to increase or decrease the drug partitioning into the bilayers. Bacteriorhodopsin proteoliposomes containing cholesterol combined the effects of the protein and the cholesterol and approached the partitioning properties of red blood cell membranes. For positively charged drugs the correlation between calculated intestinal permeability and log Ks was fair for both liposomes and bacteriorhodopsin-cholesterol proteoliposomes. Detailed modeling of solute partitioning into biological membranes may require an extensive knowledge of their structures.

Advantages of quantitative affinity chromatography for the analysis of solute interaction with membrane proteins

The use of membrane proteins as chromatographic stationary phases for the quantitation of biospecific interaction between the proteins and solutes is reviewed. This method is one among the few where a membrane protein is immobilized for repeated analyses of solute binding. To our knowledge, five transmembrane proteins have been immobilized in chromatographic matrices: the glucose and nucleoside transporters from human red blood cells, the photosynthetic reaction center from Rhodobacter sphaeroides, the nicotinic acetylcholine receptor from rat brain and a recombinant P-glycoprotein. Proteoliposomes and membrane vesicles have thereby been entrapped in size-exclusion beads, such as Superdex 200, and membrane proteins have been adsorbed on 'immobilized artificial membrane' monolayers of lipid analogs grafted to silica beads. Encouragingly, immobilized glucose transporter and P-glycoprotein showed constant interactant affinities for months. Analysis is done in the frontal mode at equilibrium because there is no separation between bound and free ligand. Both the affinity constant, which generally coincides with the corresponding constant determined by use of nonchromatographic methods, and the amount of active binding sites are obtained. The method has been successfully applied to functional analysis of membrane proteins in cells or reconstituted in lipid mono- or bilayers, screening of low-molecular interactants, investigation of protein-protein interaction and studies of effects of physico-chemical parameters on solute-protein interaction. The analyses require sensitive detection of the analyte and matching between amount of binding sites and affinity.