Cytochalasin B interferes with conformational changes of the human erythrocyte glucose transporter induced by internal and external sugar binding

Interactions of androgens, green tea catechins and the antiandrogen flutamide with the external glucose-binding site of the human erythrocyte glucose transporter GLUT1

This study investigates the effects of androgens, the antiandrogen flutamide and green tea catechins on glucose transport inhibition in human erythrocytes. These effects may relate to the antidiabetogenic effects of green tea. Testosterone, 4-androstene-3,17-dione, dehydroepiandrosterone (DHEA) and DHEA-3-acetate inhibit glucose exit from human erythrocytes with half-maximal inhibitions (Ki) of 39.2+/-8.9, 29.6+/-3.7, 48.1+/-10.2 and 4.8+/-0.98 microM, respectively. The antiandrogen flutamide competitively relieves these inhibitions and of phloretin. Dehydrotestosterone has no effect on glucose transport, indicating the differences between androgen interaction with GLUT1 and human androgen receptor (hAR). Green tea catechins also inhibit glucose exit from erythrocytes. Epicatechin 3-gallate (ECG) has a Ki ECG of 0.14+/-0.01 microM, and epigallocatechin 3-gallate (EGCG) has a Ki EGCG of 0.97+/-0.13 microM. Flutamide reverses these effects. Androgen-screening tests show that the green tea catechins do not act genomically. The high affinities of ECG and EGCG for GLUT1 indicate that this might be their physiological site of action. There are sequence homologies between GLUT1 and the ligand-binding domain (LBD) of hAR containing the amino-acid triads Arg 126, Thr 30 and Asn 288, and Arg 126, Thr 30 and Asn 29, with similar 3D topology to the polar groups binding 3-keto and 17-beta OH steroid groups in hAR LBD. These triads are appropriately sited for competitive inhibition of glucose import at the external opening of the hydrophilic pore traversing GLUT1.

Beta-very low density lipoprotein induces triglyceride accumulation through receptor mediated endocytotic pathway in 3T3-L1 adipocytes

To elucidate the mechanism of triglyceride (TG) accumulation in adipocytes induced by TG-rich lipoproteins, we examined the effect of beta-very low density lipoprotein (beta-VLDL) on TG accumulation in 3T3-L1 adipocytes. Beta-VLDL did not induce TG accumulation in 3T3-L1 preadipocytes but in 3T3-L1 adipocytes. TG accumulation was significantly inhibited by cytochalasin B, an inhibitor of receptor mediated endocytosis. In contrast, cytochalasin B did not inhibit free fatty acid induced TG accumulation in adipocytes. The binding of [125I]beta-VLDL to preadipocytes was inhibited completely by both beta-VLDL and LDL. In sharp contrast, the binding of [125I]beta-VLDL to adipocytes was inhibited completely by beta-VLDL, but partially by LDL. The VLDL receptor mRNA was only expressed in adipocytes. These results suggest that beta-VLDL induced TG accumulation in adipocytes may be mediated through the VLDL receptor pathway.

Influence of cycloheximide-mediated downregulation of glucose transport on TNF alpha-induced apoptosis

Enhancement of cellular sensitivity to TNF alpha-induced apoptosis by cycloheximide (CX) has been attributed to its quality as an inhibitor of protein synthesis, presumably by prevention of the synthesis of short-lived death antagonists. CX is also known to interfere with glucose transport, which in turn influences cell death. Hexose uptake, expression of glucose transporter (Glut) mRNAs and proteins, and other related factors were therefore examined upon induction of apoptosis with TNF alpha and CX in breast cancer cell lines. In the early phase of apoptosis, a dramatic decrease in glucose transport was observed, preceded by stimulation of Glut 1 and 3 mRNAs. Transport downregulation was also detectable upon incubation with CX alone, albeit to a lesser extent. With the doses used, TNF alpha had no such effect. Protein synthesis was inhibited to the same degree in TNF alpha/CX-treated apoptotic cells compared to viable CX-treated cells. Diminished hexose uptake was associated with decreased Vmax, while Glut affinity remained unaffected. As there was no evidence for changes in total cellular Glut content or for Glut translocation from the plasma membrane, a diminished intrinsic activity of Gluts must be postulated. In conclusion, CX is proposed to contribute to TNF alpha-induced apoptosis predominantly by interference with glucose transport; the exact nature of this effect remains to be elucidated.

Asparagine 394 in putative helix 11 of the galactose-H+ symport protein (GalP) from Escherichia coli is associated with the internal binding site for cytochalasin B and sugar

The galactose-H+ symport protein (GalP) of Escherichia coli is very similar to the human glucose transport protein, GLUT1, and both contain a highly conserved Asn residue in predicted helix 11 that is different in a cytochalasin B-resistant member of this sugar transport family (XylE). The role of the Asn394 residue (which is predicted to be in putative trans-membrane alpha-helix 11) in the structure/activity relationship of the D-galactose-H+ symporter (GalP) was therefore assessed by measuring the interaction of sugar substrates and the inhibitory antibiotics, cytochalasin B, and forskolin with the wild-type and Asn394 --> Gln mutant proteins. Steady-state fluorescence quenching experiments show that the mutant protein binds cytochalasin B with a Kd 37-53-fold higher than the wild type. This low affinity binding was not detected with equilibrium binding or photolabeling experiments. In contrast, the mutant protein binds forskolin with a Kd similar to that of the wild type and is photolabeled by 3-125I-4-azido-phenethylamido-7-O-succinyl-desacetyl-forskolin. The mutant protein displays an increased amount of steady-state fluorescence quenching with the binding of forskolin, suggesting that the substitution of the Asn residue has altered the environment of a tryptophan, probably Trp395, in a conformationally active region of the protein. Time-resolved fluorescence measurements on the mutant protein provided association and dissociation rate constants (k2 and k-2), describing the initial interaction of cytochalasin B to the inward-facing binding site (Ti), that are decreased (9-fold) and increased (4.9-fold) compared with the wild type. This yielded a dissociation constant (K2) for cytochalasin B to the inward-facing binding site 44-fold higher than that of the wild type. The binding of forskolin gave values for k2 and k-2 3.9- and 3.6-fold lower, respectively, yielding a K2 value for Ti similar to that of the wild type. The low overall affinity (high Kd) of the mutant protein for cytochalasin B is due mainly to a disruption in binding to the Ti conformation. It is proposed that Asn394 forms either a direct binding interaction with cytochalasin B or is part of the immediate environment of the binding site and that Asn394 is in the immediate environment, but not part, of the forskolin binding site. The ability of the mutant protein to catalyze energized transport is only mildly impaired with 4.8- and 2.1-fold reduction in Vmax/Km values for D-galactose and D-glucose, respectively. In stark contrast, the overall Kd describing binding of D-galactose and D-glucose to the inward-facing conformation of the mutant and their subsequent translocation across the membrane is substantially increased (64-fold for D-galactose and 163.3-fold for D-glucose). These data indicate that Asn394 is associated with both the cytochalasin B and internal sugar binding sites. This conclusion is also supported by data showing that the sugar specificity of the mutant protein has been altered for D-xylose. This work powerfully illustrates how comparisons of the aligned amino acid sequences of homologous membrane proteins of unknown structure and characterization of their phenotypes can be used to map substrate and ligand binding sites.

The role of tryptophans 371 and 395 in the binding of antibiotics and the transport of sugars by the D-galactose-H+ symport protein (GalP) from Escherichia coli

The interactions between the D-galactose-H+ symporter (GalP) from Escherichia coli and the inhibitory antibiotics, cytochalasin B and forskolin, and the substrates, D-galactose and H+, have been investigated for the wild-type protein and the mutants Trp-371-->Phe and Trp-395-->Phe, so that the roles of these residues in the structure-activity relationship could be assessed. Neither mutation prevented photolabeling by either [4-3H]cytochalasin B or by 3-[125I]iodo-4-azidophenethyl-amido-7-O-succinyldesacetylforskolin ([125I]APS-forskolin). However, measurements of protein fluorescence show that both residues are in structural domains, the conformations of which are perturbed by the binding of cytochalasin B or forskolin. Moreover, both mutations cause a substantial decrease in the affinity of the inward-facing site of the GalP protein for cytochalasin B, 10- and 43-fold, respectively, but have little effect upon the affinity of this site for forskolin, 0.8- and 2.6-fold reductions, respectively. Both these mutations change the equilibrium between the putative outward- (T1) and inward-facing (T2) conformations, so that the inward-facing form is more favored. They also stabilize a different conformational state, "T3-antibiotic," in which the initial interactions between the protein and antibiotics are tightened. Overall, this has the effect of compensating for the reduction in affinity for cytochalasin B, so that the respective overall Kd values are 0.74- and 3.5-fold that of the wild type, while causing a slight increase, 1.5- and 3.2-fold, respectively, in affinity of the mutants for forskolin. The Trp-371-->Phe mutation causes a 15-fold reduction in the affinity of the inward-facing site for D-galactose, suggesting that this residue forms part of the sugar binding site. In contrast, the Trp-395-->Phe mutation has no effect upon the affinity of the inward-facing site for D-galactose. These effects may be related to the reduction in galactose-H+ symport activity only in the Trp-371-->Phe mutant, although it still effects active transport to the same extent as the Trp395-->Phe mutant. However, there is a 10-20-fold increase in the Km values for energized transport of D-galactose for both mutants.

The role of cysteine residues in glucose-transporter-GLUT1-mediated transport and transport inhibition

The role of cysteine residues in transport function of the glucose transporter GLUT1 was investigated by a mutagenesis-expression strategy. Each of the six cysteine residues was individually replaced by site-directed mutagenesis. Expression of the heterologous wild-type or mutant glucose transporters and transport measurements at two hexose concentrations (50 microM and 5 mM) were undertaken in Xenopus oocytes. The catalytic activity of GLUT1 was retained, despite substitution of each single cysteine residue, which indicated that no individual residue is essential for hexose transport. This finding questions the involvement of oligomerization or intramolecular stabilization by a single disulphide bond as a prerequisite for transporter activation under basal conditions. Application of the impermeant mercurial thiol-group-reactive reagent p-chloromercuribenzenesulphonate (pCMBS) to the external or internal surface of plasma membrane demonstrated that cysteine-429, within the sixth external loop, and cysteine-207, at the beginning of the large intracellular loop which connects transmembrane segments 6 and 7, are the residues which are involved in transport inhibition by impermeant thiol-group-reactive reagents from either side of the cell. These data support the predicted membrane topology of the transport protein by transport measurements. If residues other than the cysteines at positions 429 or 207 are exposed to either side of the plasma membrane by conformational changes, they do not contribute to the transport inhibition by pCMBS. Application of pCMBS to one side of the plasma membrane also inhibited transport from the opposite direction, most likely due to the hindrance of sugar-induced interconversion of transporter conformation.

The kinetics and thermodynamics of the binding of cytochalasin B to sugar transporters

The kinetics of the binding of cytochalasin B to the proton-linked L-arabinose (AraE) and D-galactose (GalP) symporters from Escherichia coli and to the human erythrocyte glucose transporter (GLUT1) have been investigated by exploiting the changes in protein fluorescence that occur upon binding the ligand. Steady-state measurements yielded Kd values of 1.1, 1.9 and 0.14 microM for the AraE, GalP and GLUT1 proteins, respectively. The association and dissociation rate constants for the binding of cytochalasin B have been determined by stopped-flow spectroscopy. In each case, the apparent Kd was calculated from the corresponding rate constants, yielding values of 1.5, 0.4 and 1.6 microM for AraE, GalP and GLUT1, respectively. The differences between these apparent Kd values and those measured by fluorescence titration is interpreted in terms of the following three step mechanism where CB represents cytochalasin B: [formula: see text] The transporter is proposed to alternate between two different conformational forms (T1 and T2), with cytochalasin B binding only to the T2 conformation, to induce a further conformational transition of the transporter to the T3 form. The values for the overall dissociation constants show that the T1 conformation is favoured by AraE and GalP in the absence of ligands, but the T2 conformation is favoured by GLUT1. Thus, the binding of cytochalasin B to GLUT1 alters the equilibrium towards the T3(CB) conformational state, producing the observed tight binding, in contrast to the changes in the equilibrium observed with the binding of cytochalasin B to AraE and GalP. A thermodynamic analysis of these conformational transitions has been performed. The T1 and T2 conformations may represent transporter states in which the binding site is facing outwards and inwards, respectively.

Ligand-induced conformational changes modify proteolytic cleavage of the adipocyte insulin-sensitive glucose transporter

The transport conformation of the human erythrocyte glucose transporter (GLUT1) modifies rates of proteolytic cleavage of this protein by a variety of enzymes. We investigated the effects of ligand-induced conformational change on the susceptibility to enzymic cleavage of the insulin-sensitive rat adipocyte glucose transporter (GLUT4). A GLUT4-enriched slow sedimenting microsomal fraction was prepared from basal adipocytes and subjected to PAGE and immunoblotting. The GLUT4 protein was detected in these immunoblots with a C-terminal-specific antiserum as an M(r)-46,000-50,000 doublet. GLUT1 protein was not detected by a GLUT1-specific antiserum in these membranes. Tryptic digestion caused loss of the GLUT4 signal in immunoblots in a time- and concentration-dependent fashion. Low-M(r) membrane-bound fragments were not observed in electrophoretic gels, whether detection was attempted by immunoblotting or by counting radioactivity in gel slices following photolabelling with [3H]cytochalasin B. Transport-specific ligands known to induce an outward-facing conformation in the human erythrocyte GLUT1 protein retarded cleavage of the GLUT4 protein by submaximal concentrations of trypsin, whereas ligands known to induce an inward-facing conformation increased the extent of cleavage. The transported substrate D-glucose retarded tryptic cleavage of GLUT4. This result contrasts with the known behaviour of GLUT1, in which D-glucose accelerates cleavage. Cleavage of GLUT4 by thermolysin was also retarded by the outward-binding analogue 4,6-O-ethylidene glucose. These results show that the conformational sensitivity to proteolysis of GLUT4 mirrors that of GLUT1, except that the glucose-loaded GLUT4 has a different steady-state configuration, which may reflect underlying kinetic differences between the two proteins.

Tryptic digestion of the human erythrocyte glucose transporter: effects on ligand binding and tryptophan fluorescence

The conformation of the human erythrocyte glucose transport protein has been shown to determine its susceptibility to enzymatic cleavage on a large cytoplasmic loop. We took the converse approach and investigated the effects of tryptic digestion on the conformational structure of this protein. Exhaustive tryptic digestion of protein-depleted erythrocyte ghosts decreased the affinity of the residual transporter for cytochalasin B by 3-fold but did not affect the total number of binding sites. Tryptic digestion also increased the affinity of the residual transporter for D-glucose and inward-binding sugar phenyl beta-D-glucopyranoside but decreased that for the outward-binding 4,6-O-ethylidene glucose. These results suggest that tryptic cleavage stabilized the remaining transporter in an inward-facing conformation, but one with decreased affinity for cytochalasin B. The steady-state fluorescence emission scan of the purified reconstituted glucose transport protein was unaffected by tryptic digestion. Addition of increasing concentrations of potassium iodide resulted in linear Stern-Volmer plots, which were also unaffected by prior tryptic digestion. The tryptophan oxidant N-bromosuccinimide was investigated to provide a more sensitive measure of tryptophan environment. This agent irreversibly inhibited 3-O-methylglucose transport in intact erythrocytes and cytochalasin B binding in protein-depleted ghosts, with a half-maximal effect observed for each activity at about 0.3-0.4 nM. Treatment of purified glucose transport protein with N-bromosuccinimide resulted in a time-dependent quench of tryptophan fluorescence, which was resolved into two components by nonlinear regression using global analysis. Tryptic digestion retarded the rate of oxidation of the more slowly reacting class of tryptophans. (ABSTRACT TRUNCATED AT 250 WORDS)

The 12-transmembrane helix transporters

From the hydropathic profiles of their amino acid sequences many transport proteins are conceived to comprise 12-transmembrane alpha-helices. In only a few examples, however, is there genetical and/or biochemical evidence to support the 12-helix structure or illuminate the molecular mechanism of the transport process. A number of these transport proteins occur in evolutionarily related families, and sometimes superfamilies, indicating divergent evolution of the 12-helix structure. Other individual members or families of transport proteins are sufficiently different in amino acid sequence for their evolution to have taken place by convergence from independent ancestral origins.

Equilibrium and transient kinetic studies of the binding of cytochalasin B to the L-arabinose-H+ symport protein of Escherichia coli. Determination of the sugar binding specificity of the L-arabinose-H+ symporter

The kinetics of the binding of cytochalasin B to the L-arabinose-H+ symport protein of Escherichia coli have been investigated, using a strain that over-produces the symport protein in the cytoplasmic membrane. Equilibrium binding studies revealed a single set of binding sites (2.9-8.9 nmol/mg protein) with a Kd of 0.7-1.0 microM at 22 degrees C. It proved possible to follow the transient kinetics of cytochalasin B binding by measuring the changes in the fluorescence of the L-arabinose-H+ symporter upon binding the ligand, by stopped-flow fluorescence spectroscopy. The association and dissociation rate constants thus determined were confirmed by rapid filtration measurements, using [3H]cytochalasin B, yielding values of 4.5-6.5 microM-1.s-1 and 4-5 s-1, respectively, consistent with Kd values obtained by measuring equilibrium binding of [3H]cytochalasin B by dialysis at 22 degrees C. Titration of the protein fluorescence with cytochalasin B yielded a similar binding site concentration and Kd value to those obtained in equilibrium binding studies. All the measurements of binding site concentration are consistent with a stoichiometry of 1 mol cytochalasin B binding sites/mol L-arabinose-H+ symport protein. Inhibition of both the rate and equilibrium binding of cytochalasin B by sugars indicated the following order of substrate binding 5-thio-D-glucose > D-fucose > L-arabinose > 6-deoxy-6-fluoro-D-galactose > D-xylose approximately 6-deoxy-D-glucose > D-galactose > D-glucose > D-ribose. Neither D-arabinose nor L-fucose had any significant inhibitory effect upon cytochalasin B binding.

Monitoring conformational change in the human erythrocyte glucose carrier: use of a fluorescent probe attached to an exofacial carrier sulfhydryl

Several fluorescent sulfhydryl reagents were tested as probes for assessing substrate-induced conformational change of the human erythrocyte glucose carrier. Of these, 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (Mal-ANS) inhibited 3-O-methylglucose transport most strongly and specifically labeled a previously characterized exofacial sulfhydryl on the glucose carrier. Analysis of equilibrium cytochalasin B binding in cells treated with Mal-ANS suggested that the inhibition of transport was due to a partial channel-blocking effect, and not to competition for the substrate binding site or to hindrance of carrier conformational change. In purified glucose carrier prepared from cells labeled on the exofacial sulfhydryl with Mal-ANS, a blue shift in the peak of fluorescence indicated that the fluorophore was in a relatively hydrophobic environment. Mal-ANS fluorescence in such preparations was quenched by ligands with affinity for the outward-facing carrier (ethylidene glucose, D-glucose, and maltose), but not by inhibitors considered to bind to the inward-facing carrier conformation (cytochalasin B or phenyl beta-D-glucoside). The effect of ethylidene glucose appeared to be related to an interaction with the glucose carrier, since the concentration dependence of ethylidene glucose-induced quench correlated well with the ability of the sugar analog to inhibit cytochalasin B binding to intact cells. The hydrophilic quenchers iodide and acrylamide decreased carrier-bound Mal-ANS fluorescence, resulting in downward-curving Stern-Volmer plots. Whereas ethylidene glucose enhanced iodide-induced quench, it had no effect on that of acrylamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic analysis of the liver-type (GLUT2) and brain-type (GLUT3) glucose transporters in Xenopus oocytes: substrate specificities and effects of transport inhibitors

We have expressed the human isoforms of the liver-type (GLUT2) and brain-type (GLUT3) facilitative glucose transporters in oocytes from Xenopus laevis via injection of in vitro transcribed mRNA. As reported previously [Gould, Thomas, Jess and Bell (1991) Biochemistry 30, 5139-5145], GLUT2 mediates the transport of fructose and galactose, and GLUT3 mediates the transport of galactose. We have examined the effects of D-glucose, D-fructose and maltose on deoxyglucose transport in oocytes expressing GLUT2, and D-glucose, D-galactose and maltose on deoxyglucose transport in oocytes expressing GLUT3, and show that each sugar is a competitive inhibitor of transport. Moreover, D-glucose and maltose competitively inhibit fructose transport by GLUT2 and galactose transport by GLUT3, indicating that the transport of the alternative substrates for these transporters is likely to be mediated by the same outward-facing sugar-binding site used by glucose. Cytochalasin B is a non-competitive inhibitor of glucose transport by the well-characterized GLUT1 isoform. We show here that cytochalasin B is also a non-competitive inhibitor of the transport of deoxyglucose and alternative substrates by GLUT2 and GLUT3 expressed in oocytes. Km and Ki values for each substrate and inhibitor are presented for each isoform, together with further analysis of the binding sites for alternative substrates for these transporter isoforms.