Photolabeling of erythrocyte and adipocyte hexose transporters using a benzophenone derivative of bis(D-mannose)

Chemical biology probes of mammalian GLUT structure and function

The structure and function of glucose transporters of the mammalian GLUT family of proteins has been studied over many decades, and the proteins have fascinated numerous research groups over this time. This interest is related to the importance of the GLUTs as archetypical membrane transport facilitators, as key limiters of the supply of glucose to cell metabolism, as targets of cell insulin and exercise signalling and of regulated membrane traffic, and as potential drug targets to combat cancer and metabolic diseases such as type 2 diabetes and obesity. This review focusses on the use of chemical biology approaches and sugar analogue probes to study these important proteins.

Photoaffinity probes for studying carbohydrate biology

Carbohydrates and carbohydrate-containing biomolecules engage in binding events that underlie many essential biological processes. Yet these carbohydrate-mediated interactions are often poorly characterized, due to their low affinities and heterogenous natures. The use of photocrosslinking functional groups offers a way to photochemically capture carbohydrate-containing complexes, which can be isolated for further analysis. Here we survey progress in the synthesis and use of carbohydrate-based photoprobes, reagents that incorporate carbohydrates or their analogs, photocrosslinking moieties, and affinity purification handles. Carbohydrate photoprobes, used in combination with modern mass spectrometry methods, can provide important new insights into the cellular roles of carbohydrates and glycosylated molecules.

Kinetics of GLUT4 trafficking in rat and human skeletal muscle

OBJECTIVE

In skeletal muscle, insulin stimulates glucose transport activity three- to fourfold, and a large part of this stimulation is associated with a net translocation of GLUT4 from an intracellular compartment to the cell surface. We examined the extent to which insulin or the AMP-activated protein kinase activator AICAR can lead to a stimulation of the exocytosis limb of the GLUT4 translocation pathway and thereby account for the net increase in glucose transport activity.

RESEARCH DESIGN AND METHODS

Using a biotinylated photoaffinity label, we tagged endogenous GLUT4 and studied the kinetics of exocytosis of the tagged protein in rat and human skeletal muscle in response to insulin or AICAR. Isolated epitrochlearis muscles were obtained from male Wistar rats. Vastus lateralis skeletal muscle strips were prepared from open muscle biopsies obtained from six healthy men (age 39 +/- 11 years and BMI 25.8 +/- 0.8 kg/m2).

RESULTS

In rat epitrochlearis muscle, insulin exposure leads to a sixfold stimulation of the GLUT4 exocytosis rate (with basal and insulin-stimulated rate constants of 0.010 and 0.067 min(-1), respectively). In human vastus lateralis muscle, insulin stimulates GLUT4 translocation by a similar sixfold increase in the exocytosis rate constant (with basal and insulin-stimulated rate constants of 0.011 and 0.075 min(-1), respectively). In contrast, AICAR treatment does not markedly increase exocytosis in either rat or human muscle.

CONCLUSIONS

Insulin stimulation of the GLUT4 exocytosis rate constant is sufficient to account for most of the observed increase in glucose transport activity in rat and human muscle.

Insulin and Contraction Stimulate Exocytosis, but Increased AMP-activated Protein Kinase Activity Resulting from Oxidative Metabolism Stress Slows Endocytosis of GLUT4 in Cardiomyocytes

Stimulations of glucose transport produced by insulin action, contraction, or through a change in cell energy status are mediated by separate signaling pathways. These are the wortmannin-sensitive phosphatidylinositol 3-kinase pathway leading to the intermediate Akt and the wortmannin-insensitive AMP-activated protein kinase (AMPK) pathway. Electrical stimulation of cardiomyocytes produced a rapid, insulin-like, wortmannin-sensitive stimulation of glucose transport activity, but this occurred without extensive activation of Akt. Although AMPK phosphorylation was increased by contraction, this response was not wortmannin-inhibitable and consequently did not correlate with the wortmannin sensitivity of the transport stimulation. Oxidative metabolism stress due to hypoxia or treatment with oligomycin led to increased AMPK activity with a corresponding increase in glucose transport activity. We show here that these separate signaling pathways converge on GLUT4 trafficking at separate steps. The rate of exocytosis of GLUT4 was rapidly stimulated by insulin, but insulin treatment did not alter the endocytosis rate. Like insulin stimulation, electrical stimulation of contraction led to a stimulation of GLUT4 exocytosis without any marked change in endocytosis. By contrast, after oxidative metabolism stress, no stimulation of GLUT4 exocytosis occurred; instead, this treatment led to a reduction in GLUT4 endocytosis.

Development of high-affinity ligands and photoaffinity labels for the D-fructose transporter GLUT5

The GLUT5 transporter catalyses the specific uptake of D-fructose and can accept this hexose in its furanose and pyranose ring forms. The transporter does not accept fructose epimers and has very limited tolerance of bulky groups substituted at the 2-, 3-, 4- and 5-OH positions [Tatibouët, Yang, Morin and Holman (2000) Bioorg. Med. Chem. 8, 1825-1833]. To further explore whether bulky groups can be tolerated at the primary OH positions, a D-fructose analogue with an allylamine group substitution to replace the 1-OH group was synthesized and was found to be quite well tolerated ( K (i)=27.1 mM). However, this analogue occurs in multiple ring forms. By contrast, 2,5-anhydro-D-mannitol is a symmetrical molecule that occurs only in a furanose ring form in which C-1 and C-6 are equivalent. We have therefore synthesized new 2,5-anhydro-D-mannitol analogues (substituted at the equivalent of the 6-OH of D-fructose) and from studies in Chinese hamster ovary cells expressing GLUT5 cells report that (i) the allylamine derivative of 2,5-anhydro-D-mannitol is well tolerated ( K (i)=2.66 mM); (ii) introduction of a di-nitrophenyl-substituted secondary amine group enhances affinity ( K (i)=0.56 mM); (iii) introduction of amide-linked biotinylated photolabel moieties is possible without loss of affinity relative to 2,5-anhydro-D-mannitol but a small secondary amine spacer between the biotinylated photolabelling moiety and the fructofuranose ring increases affinity (fructose photolabel 2; K (i)=1.16 mM); (iv) introduction of a hydrophilic tartarate spacer between biotin and the diazirine photoreactive groups can be accomplished without reduction in affinity and (v) photoactivation of biotinylated fructose photolabels leads to specific biotin tagging of GLUT5. These data suggest that substitution of a secondary amine group (-NH) to replace the C-6 (or C-1) -OH of 2,5-anhydro-D-mannitol results in compounds of high affinity; the affinity is enhanced over 10-fold compared with D-fructose.

Improvement in the properties of 3-phenyl-3-trifluoromethyldiazirine based photoreactive bis-glucose probes for GLUT4 following substitution on the phenyl ring

We have developed two novel 3-phenyl-3-trifluoromethyldiazirinyl bis-glucose derivatives to investigate the properties of the adipocyte glucose transporter GLUT4. These compounds were substituted by electron-withdrawing (iodo and nitro) groups on the aromatic ring of 3-phenyl-3-trifluoromethyldiazirine photophore and were found to be more photosensitive than compounds without such substituents. The compounds were used as inhibitors of insulin-stimulated glucose transport activity in order to assess half-maximal inhibition or relative affinity values for GLUT4. The affinities were found to be 60-130 times higher than the parent compound D-glucose. Because of the increased photo-reactivity and high affinity these compounds will be useful in studies directed at further elucidation of GLUT4 function.

Synthesis of biotinylated bis(D-glucose) derivatives for glucose transporter photoaffinity labelling

New diazirine based bis-glucose derivatives for tagging glucose transporters have been synthesised. These included two biotinylated compounds linked either by an aminocaproate or by a cleavable dithiol link. These compounds have been derivatised via a key skeleton compound that can be easily used for introduction of additional tags. Studies on the erythrocyte glucose transporter (GLUT1) and the insulin-stimulated adipose cell transporter (GLUT4) have revealed the biotinylated photoreactive bis-glucose compounds are effective labelling reagents.

Cell-surface recognition of biotinylated membrane proteins requires very long spacer arms: an example from glucose-transporter probes

Glucose transporters (GLUTs) can be photoaffinity labelled by (diazirinetrifluoroethyl)benzoyl-substituted glucose derivatives and the adduct can be recognised, after detergent solubilisation of membranes, by using streptavidin-based detection systems. However, in intact cells recognition of photolabelled GLUTs by avidin and anti-biotin antibodies only occurs if the bridge between the photoreactive and the biotin moieties has a minimum of 60--70 spacer atoms. We show that a suitably long bridge can be synthesised with a combination of polyethylene glycol and tartarate groups and that introduction of these spacers generates hydrophilic products that can be cleaved with periodate. Introduction of the very long spacers does not appreciably reduce the affinity of interaction of the probes with the transport system.

Novel photoreactive cinnamic acid analogues to elucidate phenylalanine ammonia-lyase

4-[3-(Trifluoromethyl) diazirinyl] cinnamic acid derivatives were synthesized to elucidate properties of phenylalanine ammonia-lyase (PAL). 2-Methoxy and 2-biotinylated alkoxy compounds have inhibitory activity on the formation of phenylalanine from cinnamic acid. Specific photolabeling of the enzyme was detected using biotinylated derivatives without the use of radioisotopes. The results indicated that the 4-[3-(trifluoromethyl) diazirinyl] skeleton will be a suitable photoreactive compound to elucidate regulation of phenylpropanoid biosynthesis.

Quantitative methods for measuring the insulin-regulatable glucose transporter (Glut4)

This review article describes various quantitation methods for the insulin-regulatable glucose transporter (Glut4). Several methods including reconstituted glucose transport, cytochalasin B binding assays, immunocytochemistry, immunoblots, ELISA, and the more recently developed exofacial labels are discussed. Since Glut4 translocates from an intracellular compartment to the plasma membrane in response to the action of insulin, it is of particular interest to measure Glut4 changes in the membrane fractions. Hence, the measurement of Glut4 commonly involves the isolation of cell membranes using subcellular fractionation in combination with one of the quantitation methods. The limitations of each quantitation method due to the use of subcellular fractionation are discussed in this article. As well, the advantages and disadvantages in terms of isoform specificity and technical difficulties of each method are presented.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Benzophenone photophores in biochemistry

The photoactivatable aryl ketone derivatives have been rediscovered as biochemical probes in the last 5 years. The expanding use of benzophenone (BP) photoprobes can be attributed to three distinct chemical and biochemical advantages. First, BPs are chemically more stable than diazo esters, aryl azides, and diazirines. Second, BPs can be manipulated in ambient light and can be activated at 350-360 nm, avoiding protein-damaging wavelengths. Third, BPs react preferentially with unreactive C-H bonds, even in the presence of solvent water and bulk nucleophiles. These three properties combine to produce highly efficient covalent modifications of macromolecules, frequently with remarkable site specificity. This Perspectives includes a brief review of BP photochemistry and a selection of specific applications of these photoprobes, which address questions in protein, nucleic acid, and lipid biochemistry.

Regulation of glucose uptake by stressed cells

Lactate production by BHK cells is stimulated by arsenite, azide, or by infection with Semliki Forest virus (SFV). In the case of arsenite or SFV infection, the increase correlates approximately with the increase in glucose transport as measured by uptake of [3H] deoxy glucose (dGlc); in the case of azide, the increase in lactate production exceeds that of glucose transport. Hence glucose utilization by BHK cells and its stimulation by anaerobic and other types of cellular stress is controlled at least in part at the level of glucose transport. The glucose uptake by BHK cells is also stimulated by serum and by glucose deprivation. In these circumstances, as with arsenite, stimulation is reversible, with t1/2 of 1-2 hours; stimulation is compatible with a translocation of the glucose transporter protein between an intracellular site and the plasma membrane (shown here for serum and previously for arsenite). The surface binding and rate of internalization of [125I]-labelled transferrin and [125I] alpha 2-macroglobulin was studied to determine whether changes in glucose transport are accompanied by changes in the surface concentration or rate of internalization of membrane proteins. The findings indicate that changes in glucose transport do not reflect a consistent and general redistribution of membrane receptors. Taken together, the results are compatible with the proposal that BHK cells exposed to stimuli like insulin or serum, or to stresses like arsenite, azide, SFV infection, or deprivation of glucose, respond in the same manner: namely, by an increased capacity to transport glucose brought about by reversible and specific translocation of the transporter protein from an (inactive) intracellular site to the plasma membrane.

Inhibition of the Na+/I- symporter by harmaline and 3-amino-1-methyl-5H-pyrido(4,3-b)indole acetate in thyroid cells and membrane vesicles

Novel inhibitors of the Na+/I- symporter were identified using rat-thyroid-derived FRTL-5 cells and sealed vesicles from calf thyroid as model systems. Na(+)-dependent 125I- uptake was inhibited by the hallucinogenic drug harmaline and by a chemically related convulsive agent, 3-amino-1-methyl- 5H-pyrido(4,3-b)indole acetate (TRP-P-2). TRP-P-2 (Ki = 0.25 mM) was tenfold more effective as an inhibitor than harmaline (Ki = 4.0 mM). Inhibition by TRP-P-2 was competitive with respect to Na+ and was fully reversible. Although TRP-P-2 is a relatively low-affinity inhibitor, its affinity for the Na+ site of the Na+/I- symporter is over 100 times higher than that of Na+ (Km = 50 mM). 45Ca(2+)-efflux rates in calf thyroid membrane vesicles were not affected by TRP-P-2, indicating that membrane integrity is not disrupted by the drug. These findings show that TRP-P-2 may be a potentially useful tool for the identification and characterization of the Na+/I- symporter.

Determination of the rates of appearance and loss of glucose transporters at the cell surface of rat adipose cells

We have used an impermeant bis-mannose compound (2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis-(D-mannos+ ++- 4-yloxy)-2- propylamine; ATB-BMPA) to photolabel the glucose transporter isoforms GLUT4 and GLUT1 that are present in rat adipose cells. Plasma-membrane fractions and light-microsome membrane fractions were both labelled by ATB-BMPA. The labelling of GLUT4 in the plasma membrane fraction from insulin-treated cells was approximately 3-fold higher than that of basal cells and corresponded with a decrease in the labelling of the light-microsome fraction. In contrast with this, the cell-surface labelling of GLUT4 from insulin-treated intact adipose cells was increased approximately 15-fold above basal levels. In these adipose cell preparations, insulin stimulated glucose transport activity approximately 30-fold. Thus the cell-surface labelling, but not the labelling of membrane fractions, closely corresponded with the stimulation of transport. The remaining discrepancy may be due to an approx. 2-fold activation of GLUT4 intrinsic transport activity. We have studied the kinetics of trafficking of transporters and found the following. (1) Lowering the temperature to 18 degrees C increased basal glucose transport and levels of cell-surface glucose transporters by approximately 3-fold. This net increase in transporters probably occurs because the process of recruitment of transporters is less temperature-sensitive than the process involved in internalization of cell-surface transporters. (2) The time course for insulin stimulation of glucose transport activity occurred with a slight lag period of 47 s and a t 1/2 3.2 min. The time course of GLUT4 and GLUT1 appearance at the cell surface showed no lag and a t 1/2 of approximately 2.3 min for both isoforms. Thus at early times after insulin stimulation there was a discrepancy between transporter abundance and transport activity. The lag period in the stimulation of transport activity may represent the time required for the approximately 2-fold stimulation of transporter intrinsic activity. (3) The decrease in transport activity after insulin removal occurred with a very high activation energy of 159 kJ.mol-1. There was thus no significant decrease in transport or less of cell-surface transporters over 60 min at 18 degrees C. The decrease in transport activity occurred with a t1/2 of 9-11 min at 37 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of the forskolin photolabelling site within the monosaccharide transporter of human erythrocytes

Chemical and proteolytic digestion of intact erythrocyte glucose transporter as well as purified transporter protein has been used to localize the derivatization site for the photoaffinity agent 3-[125I]iodo-4-azido-phenethylamino-7-O-succinyldeacetylforskol in [( 125I]IAPS-forskolin). Comparison of the partial amino acid sequence of the labelled 18 kDa tryptic fragment with the known amino acid sequence for the HepG2 glucose transporter confirmed that the binding site for IAPS-forskolin is between the amino acid residues Glu254 and Tyr456. Digestion of intact glucose transporter with Pronase suggests that this site is within the membrane bilayer. Digestion of labelled transporter with CNBr generated a major radiolabelled fragment of Mr approximately 5800 putatively identified as residues 365-420. Isoelectric focusing of Staphylococcus aureus V8 proteinase-treated purified labelled tryptic fragment identified two peptides which likely correspond to amino acid residues 360-380 and 381-393. The common region for these radiolabelled peptides is the tenth putative transmembrane helix of the erythrocyte glucose transporter, comprising amino acid residues 369-389. Additional support for this conclusion comes from studies in which [125I]APS-forskolin was photoincorporated into the L-arabinose/H(+)-transport protein of Escherichia coli. Labelling of this transport protein was protected by both cytochalasin B and D-glucose. The region of the erythrocyte glucose transporter thought to be derivatized with IAPS-forskolin contains a tryptophan residue (Trp388) that is conserved in the sequence of the E. coli arabinose-transport protein.

Exofacial photolabelling of the human erythrocyte glucose transporter with an azitrifluoroethylbenzoyl-substituted bismannose

The synthesis of 2-N-[4-(1'-azitrifluoroethyl)benzoyl]-1,3-bis-(D-mannos-4-++ +yloxy)-2- propylamine (ATB-BMPA) is described. This compound was used as an exofacial probe for the human erythrocyte glucose-transport system. A new method is described for directly estimating the affinity for exofacial ligands which bind to the erythrocyte glucose transporter. By using this equilibrium-binding method, the Ki for ATB-BMPA was found to be 338 +/- 37 microM at 0 degrees C and 368 +/- 59 microM at 20 degrees C. This was similar to the concentration of ATB-BMPA required to half-maximally inhibit D-galactose uptake (Ki = 297 +/- 53 microM). The new photoaffinity reagent labelled the glucose transporter in intact cells but, because of its improved selectivity, was also used to label the glucose transporter in isolated erythrocyte membranes. The ATB-BMPA-labelled glucose transporter was 80% immunoprecipitated by anti-(GLUT1-C-terminal peptide) antibody, which shows that the GLUT1 glucose transporter is the major isoform present in erythrocytes. The labelling of the glucose transporter at its exofacial site, and the adoption of an outward-facing conformation, renders the transport system resistant to thermolysin and trypsin treatment. Trypsin treatment of the unlabelled glucose transporter in erythrocyte membranes produced an 18 kDa fragment which was subsequently labelled by ATB-BMPA, but had low affinity for this exofacial ligand. This suggests that the trypsin-treated transporter adopts an inward-facing conformation. The ability of D-glucose to displace ATB-BMPA from the native transporter and from the 18 kDa trypsin fragment have been compared. The D-glucose concentration which was required to obtain half-maximal inhibition of ATB-BMPA labelling was 6-fold lower for the 18 kDa tryptic fragment.