Reconstitution of the glucose transporter from bovine heart

Acidosis mediates recurrent hypoglycemia-induced increase in ischemic brain injury in treated diabetic rats

OBJECTIVES

Cerebral ischemia is a serious possible manifestation of diabetic vascular disease. Recurrent hypoglycemia (RH) enhances ischemic brain injury in insulin-treated diabetic (ITD) rats. In the present study, we determined the role of ischemic acidosis in enhanced ischemic brain damage in RH-exposed ITD rats.

METHODS

Diabetic rats were treated with insulin and mild/moderate RH was induced for 5 days. Three sets of experiments were performed. The first set evaluated the effects of RH exposure on global cerebral ischemia-induced acidosis in ITD rats. The second set evaluated the effect of an alkalizing agent (Tris-(hydroxymethyl)-aminomethane: THAM) on ischemic acidosis-induced brain injury in RH-exposed ITD rats. The third experiment evaluated the effect of the glucose transporter (GLUT) inhibitor on ischemic acidosis-induced brain injury in RH-exposed ITD rats. Hippocampal pH and lactate were measured during ischemia and early reperfusion for all three experiments. Neuronal survival in Cornu Ammonis 1 (CA1) hippocampus served as a measure of ischemic brain injury.

FINDINGS

Prior RH exposure increases lactate concentration and decreases pH during ischemia and early reperfusion when compared to controls. THAM and GLUT inhibitor treatments attenuated RH-induced increase in ischemic acidosis. GLUT inhibitor treatment reduced the RH-induced increase in lactate levels. Both THAM and GLUT inhibitor treatments significantly decreased ischemic damage in RH-exposed ITD rats.

CONCLUSIONS

Ischemia causes increased acidosis in RH-exposed ITD rats via a GLUT-sensitive mechanism. Exploring downstream pathways may help understand mechanisms by which prior exposure to RH increases cerebral ischemic damage.

Permeability of fructose-1,6-bisphosphate in liposomes and cardiac myocytes

Fructose-1,6-bisphosphate (FBP) helps preserve heart and other organs under ischemic conditions. Previous studies indicated that it can be taken up by various cell types. Here we extended observations from our group that FBP could penetrate artificial lipid bilayers and be taken up by cardiac myocytes, comparing the uptake of FBP to that of L-glucose. Using liposomes prepared by the freeze-thaw method, FBP entered about 200-fold slower than L-glucose. For liposomes of either soybean or egg lipids, 50 mM FBP enhanced the permeability of FBP itself, with little effect on general permeability (measured by uptake of L-glucose). In experiments with isolated cardiac myocytes at 21 degrees C, FBP uptake exceeded the uptake of L-glucose by several fold and appeared to equilibrate by 60 min. There was both a saturable component at micromolar levels and a nonsaturable component which dominated at millimolar levels. The saturable component was inhibited by Pi and by other phosphorylated sugars, though with lower affinity than FBP. Both saturable and nonsaturable uptakes were also observed at 3 degrees C. The results indicate that FBP enters myocytes not by simple penetration through the lipid bilayer, but via at least two distinct protein-dependent processes. The uptake could lead to intracellular effects important in hypothermic heart preservation.

On the regulatory role of dipeptidyl peptidase IV (=CD=adenosine deaminase complexing protein) on adenosine deaminase activity

The molecular mechanism controlling the variable activity of the malignancy marker adenosine deaminase (ADA) is enigmatic. ADA activity was found to be modulated by the membrane-bound adenosine deaminase complexing protein (CP=DPPIV=CD26). The role of lipid-protein interactions in this modulation was sought. While direct solubilization of ADA in vesicles resulted in loss of ADA activity, the binding of ADA to CP reconstituted in vesicles restored the specific activity. The activity of ADA, free or bound to CP in solution, resulted in continuous linear Arrhenius plots. However, ADA bound to reconstituted CP exhibited two breaks associated with approximately 30% increased activity, at 25 and 13 degrees C, yielding three lines with similar apparent activation energies (E(a)). Continuum solvent model calculations of the free energy of transfer of the transmembrane helix of CP from the aqueous phase into membranes of various widths show that the most favorable orientations of the helix above and below the main phase transition may be different. We suggest that the 20% change in the thickness of the bilayer below and above the main phase transition may modify the orientation of CP in the membrane, thereby affecting substrate accessibility of ADA. This could account for ADA's reduced activity associated with increased membrane fluidity in transformed vs. normal fibroblasts.

Surfactants in membrane solubilisation

An understanding of the action of many drugs requires a knowledge of how the drug reaches the site of action in a cell. A detailed knowledge of the structure and function of cell membranes is often required to understand the transport of drugs across the plasma membrane. To obtain this information proteins must be isolated. The isolation and characterisation of cell membrane proteins usually requires the solubilisation of the membrane and a method of separation of the various membrane proteins and glycoproteins. The starting point for such an investigation is the choice of a suitable surfactant (detergent) to solubilise the membrane. This review considers the range of surfactants that are available for membrane solubilisation, how surfactants interact with membranes, the part they play in the separation of integral membrane proteins and in the reconstitution of membrane proteins for functional studies. The solubilisation of specific membrane proteins and glycoproteins including the human erythrocyte anion transporter, mitochondrial porin, sarcoplasmic reticulum Ca(2+)-ATPase, the ATPase-active multidrug transporter P-glycoprotein, bacteriorhodopsin and rhodopsin are also discussed.

Characterization of glucose transport activity reconstituted from heart and other tissues

We examined several aspects of glucose transport reconstituted in liposomes, with emphasis on transporters of rat heart (mostly GLUT4) compared to those of human erythrocytes (GLUT1), and on effects of agents that modulate transport in intact cells. Several types of samples gave higher reconstituted activity using liposomes of egg lipids rather than soybean lipids. Diacylglycerol, proposed to activate transporters directly as part of the mechanism of insulin action, increased the intrinsic activity of heart transporters by only 25%, but increased the size of the reconstituted liposomes by 90%. The dipeptide Cbz-Gly-Phe-NH2 inhibited GLUT4 with a Ki of 0.2 mM, compared to 2.5 mM for GLUT1, which explains its preferential inhibition of insulin-stimulated glucose transport in adipocytes. Verapamil, which inhibits insulin- and hypoxia-stimulated glucose transport in muscle, had no effect on reconstituted transporters. Heart transporters had a higher Km for glucose uptake (13.4) than did GLUT1 (1.6 mM), in agreement with a recent study of GLUT1 and GLUT4 expressed in yeast and reconstituted in liposomes. Transporters reconstituted from heart and adipocytes were 40-70% inactivated by external trypsin, suggesting the presence of trypsin-sensitive sites on the cytoplasmic domain of GLUT4. NaCl and KCl both reduced reconstituted transport activity, but KCl had a much smaller effect on the size of the liposomes.

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.

Insulin-sensitive glucose transporter transcript levels in calf muscles assessed with a bovine GLUT4 cDNA fragment

Previous studies have shown that the expression of the insulin-sensitive glucose transporter (GLUT4) is lower in oxidative muscles than in glycolytic muscles in bovines and goats in contrast to observations in rats. Additional experiments in this work provide very strong arguments that the immunoreactive band detected does represent GLUT4 protein, which further validates our previous results. Therefore, to determine the level of regulation, the main objective of the present study was to measure GLUT4 mRNA amounts in various bovine muscles. A 241-bp fragment of the bovine GLUT4 cDNA was cloned by polymerase chain reaction (PCR). It shares 80-90% sequence identity with related sequences in other species. This PCR-amplified bovine GLUT4 probe was used to determine the distribution of GLUT4 mRNA in bovine tissues in comparison with that of GLUT1 mRNA. Moreover, GLUT4 mRNA amounts were quantified by Northern-blot analysis in heart and seven skeletal muscles with various oxidative and glycolytic activities from seven ruminant calves. GLUT4 mRNA was detected by Northern-blot analysis only in calf insulin-sensitive tissues. In contrast, GLUT1 mRNA was detected in all tissues studied except liver. GLUT4 mRNA amount was the highest in masseter and heart, which are oxidative muscles (1.67 +/- 0.16 and 1.53 +/- 0.19 units/g wet tissue weight, respectively) and the lowest in glycolytic or oxido-glycolytic muscles (0.31 +/- 0.04 to 1.00 +/- 0.09 units/g wet tissue weight; SEM, n = 7). These data and our previous results provide evidence for translational and/or post-translational control mechanisms of bovine GLUT4 protein expression in a muscle type-specific manner.

Translocation of two glucose transporters in heart: effects of rotenone, uncouplers, workload, palmitate, insulin and anoxia

Our previous studies on the acute regulation of glucose transport in perfused rat hearts were extended to explore further the mechanism of regulation by anoxia; to test the effects of palmitate, a transport inhibitor; and to compare the translocation of two glucose transporter isoforms (GLUT1 and GLUT4). Following heart perfusions under various conditions, glucose transporters in intracellular membranes were quantitated by reconstitution of transport activity and by Western blotting. Rotenone stimulated glucose uptake and decreased the intracellular contents of glucose transporters. This indicates that it activates glucose transport via net outward translocation, similarly to anoxia. However, two uncouplers of oxidative phosphorylation produced little or no effect. Increased workload (which stimulates glucose transport) reduced the intracellular contents of transporters, while palmitate increased the contents, indicating that these factors cause net translocation from or to the intracellular pool, respectively. Relative changes in GLUT1 were similar to those in GLUT4 for most factors tested. A plot of changes in total intracellular transporter content vs. changes in glucose uptake was roughly linear, with a slope of -0.18. This indicates that translocation accounts for most of the changes in glucose transport, and the basal pool of intracellular transporters is five times as large as the plasma membrane pool.

Partial purification and reconstitution of the sarcolemmal L-lactate carrier from rat skeletal muscle

Purified sarcolemmal membranes from mixed rat hindlimb muscle were solubilized with octylglucoside and the extract subjected to hydroxylapatite (HA) chromatography. Following protein elution with a sodium phosphate gradient and detergent removal by dialysis, the HA eluate was reconstituted into asolectin liposomes using a freeze-thaw procedure. Specific L-[14C]lactate transport activity eluting from the 0.2 M sodium phosphate fraction was 30-fold higher compared with native sarcolemmal vesicles (31.64 versus 1.06 nmol/min per mg). The reconstituted carrier exhibited Michaelis-Menten saturation kinetics with Km and Vmax. values of 46.2 +/- 6.6 mM and 498.7 +/- 17.2 nmol/15 s per mg respectively. L-Lactate transport activity was inhibited 57% by preincubation of proteoliposomes with 10 mM alpha-cyano-4-hydroxycinnamate, a known inhibitor of lactate transport. Analysis of the HA eluates by SDS/PAGE showed the presence of a 34 kDa band corresponding to lactate transport activity. Reconstitution of lactate transport activity eluting from the HA column, together with SDS/PAGE analysis suggests the presence of a 34 kDa polypeptide mediating sarcolemmal lactate exchange in rat skeletal muscle.

ATP does not regulate the reconstituted glucose transporter

ATP has been reported to affect glucose transport in human erythrocytes and resealed erythrocyte ghosts [Jacquez, J. A. (1983) Biochim. Biophys. Acta 727, 367-378; Jensen, M. R., & Brahm, J. (1987) Biochim. Biophys. Acta 900, 282-290]. In more detailed studies, effects of micromolar levels of ATP on transport in ghosts and inside-out vesicles, and on the fluorescence of ghosts and the purified glucose transporter [Carruthers, A. (1986) Biochemistry 25, 3592-3602; Hebert, D. N., & Carruthers, A. (1986) J. Biol. Chem. 261, 10093-10099; Carruthers, A. (1986) J. Biol. Chem. 261, 11028-11037], have been interpreted as supporting a model in which ATP regulates the catalytic properties of the transporter. Both allosteric and covalent effects of ATP were proposed; among the allosteric effects was a 60% reduction in the Km for zero-trans uptake. In order to test whether allosteric ATP regulation of the transporter occurs, we reconstituted glucose transport activity into liposomes using erythrocyte membranes without detergent treatment. The effects of ATP, present either outside, inside, or both inside and outside the liposomes, on the transport activity were examined. Effects of ATP on trypsin-treated liposomes, which have only a single orientation of active transporters, were also tested. While the model predicts activation by ATP, only inhibition was observed. This was significant only at millimolar concentrations of ATP, in contrast to the previously reported effects at micromolar levels, and was primarily on the extracellular surface of the transporter. In addition, the ATP effects on reconstituted transport were nonspecific, with similar effects produced by tripolyphosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethylidene glucose-substituted new analogue of streptozotocin cannot induce diabetes: study on the basis of structure and activity relationship

4,6-O-Ethylidene glucose (ethylidene glucose), a specific inhibitor at the outer surface of a glucose transporter in the cell membranes, substituted analogue of streptozotocin was newly synthesized. This compound did not induce diabetes in rats and also did not show cytotoxic effect on pancreatic beta cells of neonatal rats in a monolayer culture system. The reasons why such a molecule was designed and why it showed no biological effects are discussed on the basis of a structure-activity relationship. Our results afford positive evidence for the presence of a glucose transport system or a glucose transporter on pancreatic beta cells and its involvement in the action of streptozotocin on beta cells.

Effects of three proposed inhibitors of adipocyte glucose transport on the reconstituted transporter

Three compounds which inhibit glucose transport in rat adipocytes have been proposed to act directly on the glucose transporter protein. We tested these proposals by examining the effects of the compounds on the stereospecific glucose uptake catalyzed by adipocyte membrane proteins after reconstitution into liposomes. Effects on the transport activity reconstituted from human erythrocyte membranes were also examined. Glucose 6-phosphate, which was suggested to inhibit the transporter noncompetitively (Foley, J.E. and Huecksteadt, T.P. (1984) Biochim. Biophys. Acta 805, 313-316), had no effect on either type of reconstituted transporter, even when present at 5 mM on both sides of the liposomal membranes. Thus, it is unlikely to act directly on the transporter. The metalloendoproteinase substrate dipeptide Cbz-Gly-Phe-NH2, which inhibited insulin-stimulated but not basal glucose uptake in adipocytes (Aiello, L.P., Wessling-Resnick, M. and Pilch, P.F. (1986) Biochemistry 25, 3944-3950), inhibited the reconstituted erythrocyte transporter noncompetitively with a Ki of 1.5-2 mM. The inhibition of the erythrocyte transporter was identical in liposomes of soybean and egg lipid. Transport reconstituted using adipocyte membrane fractions was also inhibited by the dipeptide, with the activity from basal microsomes more sensitive than that from insulin-stimulated plasma membranes. These results indicate that the dipeptide interacts directly with the transporter, and may be a potentially useful probe for changes in transporter structure accompanying insulin action. Phenylarsine oxide, which was suggested to act directly on the adipocyte transporter (Douen, A.G., and Jones, M.N. (1988) Biochim. Biophys. Acta 968, 109-118), produced only slight (about 10%) inhibition of the reconstituted adipocyte and erythrocyte transporters, even when present at 100-200 microM and after 30 min of pretreatment. These results suggest that the major actions of phenylarsine oxide observed in adipocytes are not direct effects on the transporter, but rather effects on the pathways by which insulin regulates glucose transport activity (Frost, S.C. and Lane, M.D. (1985) J. Biol. Chem. 260, 2646-2652).

Identification of the glucose transporter in mammalian cell membranes with a 125I-forskolin photoaffinity label

The glucose transporter has been identified in a variety of mammalian cell membranes using a photoactivatable carrier-free radioiodinated derivative of forskolin, 3-[125I]iodo-4-azidophenethylamido-7-O-succinyldeacetylforskoli n ([125I]IAPS-forskolin) at 1-3 nM. The membranes that were photolabelled with [125I]IAPS-forskolin were human placental membranes, rat cortical and cerebellar synaptic membranes, rat cardiac sarcolemmal membranes, rat adipocyte plasma membranes, smooth-muscle membranes, and S49 wild-type (WT) lymphoma-cell membranes. The glucose transporter in plasma membranes prepared from the insulin-responsive rat cardiac sarcolemmal cells, rat adipocytes and smooth-muscle cells were determined to be approx. 45 kDa by SDS/polyacrylamide-gel electrophoresis (PAGE). Photolysis of human placental membranes, rat cortical and cerebellar synaptic membranes, and WT lymphoma membranes with [125I]-IAPS-forskolin, followed by SDS/PAGE, indicated specific derivatization of a broad band (43-55 kDa) in placental membranes and a narrower band (approx. 45 kDa) in synaptic membranes and WT lymphoma membranes. Digestion of the [125I]IAPS-forskolin-labelled placental and WT lymphoma membranes with endo-beta-galactosidase showed a reduction in the apparent molecular mass of the radiolabelled band to approx. 40 kDa. The membranes that were photolabelled with [125I]IAPS-forskolin and trypsin-treated produced a radiolabelled proteolytic fragment with an apparent molecular mass of 18 kDa. [125I]IAPS-forskolin is a highly effective probe for identifying low levels of glucose transporters in mammalian tissues.

Kinetic characterization and radiation-target sizing of the glucose transporter in cardiac sarcolemmal vesicles

Stereospecific glucose transport was assayed and characterized in bovine cardiac sarcolemmal vesicles. Sarcolemmal vesicles were incubated with D-[3H]glucose or L-[3H]glucose at 25 degrees C. The reaction was terminated by rapid addition of 4 mM HgCl2 and vesicles were immediately collected on glass fiber filters for quantification of accumulated [3H]glucose. Non-specific diffusion of L-[3H]glucose was never more than 11% of total D-[3H]glucose transport into the vesicles. Stereospecific uptake of D-[3H]glucose reached a maximum level by 20 s. Cytochalasin B (50 microM) inhibited specific transport of D-[3H]glucose to the level of that for non-specific diffusion. The vesicles exhibited saturable transport (Km = 9.3 mM; Vmax = 2.6 nmol/mg per s) and the transporter turnover number was 197 glucose molecules per transporter per s. The molecular sizes of the cytochalasin B binding protein and the D-glucose transport protein in sarcolemmal vesicles were estimated by radiation inactivation. These values were 77 and 101 kDa, respectively, and by the Wilcoxen Rank Sum Test were not significantly different from each other.

Reconstitution of the glucose transporter from rat skeletal muscle

As a step in the purification and characterization of the glucose transporter from rat skeletal muscle, we have reconstituted glucose transport activity in liposomes. Plasma membranes were prepared from skeletal muscle which display D-glucose reversible binding of cytochalasin B (10 pmol sites/mg protein; KD = 0.3 microM). Older rats gave a slightly lower specific activity and much lower yield of sites per g muscle than young rats. Glucose transport activity was reconstituted into liposomes by the freeze-thaw procedure using either plasma membranes directly or cholate-extracted membrane proteins; the latter gave a 50% higher specific activity. The reconstituted transport activity was stereospecific, saturable, and inhibited by cytochalasin B, phloretin, and mercuric chloride. The optimum cholate concentration for extraction and reconstitution of transport activity was about 1.5%, and the highest specific activity of reconstituted transport was seen only at low ratios of protein to lipid in the reconstitution. Chromatography on agarose lentil lectin and agarose ethanethiol doubled both the specific activity of reconstituted transport and the fraction of glucose uptake which was stereospecific. In all of these respects the results were similar to our results with the bovine heart transporter (T. J. Wheeler and M. A. Hauck, Biochim. Biophys. Acta 818, 171-182 (1985)). Our findings suggest that further purification procedures developed for the heart transporter may be applicable to the skeletal muscle transporter as well.

Reconstitution of glucose transport activity from erythrocyte membranes without detergent and its use in studying effects of ATP depletion

The direct reconstitution of unsolubilized membrane proteins by the freeze-thaw procedure avoids possible changes in properties produced by detergent solubilization and fractionation. Glucose transport activity was reconstituted using human erythrocyte membranes, with about 2/3 of the glucose uptake being stereo-specific. The highest specific activity occurred at low ratios of protein to lipid in the reconstitution, where most transport was due to liposomes containing single transporter molecules. Transporters were reconstituted with a scrambling of orientations, indicated by a 50% inactivation by added trypsin. Separation of unreconstituted protein doubled the specific activity. Similar results were obtained using the purified transporter (Wheeler, T.J. and Hinkle, P.C. (1981) J. Biol. Chem. 256, 8907-8914). The same ratio of net uptake to equilibrium exchange was observed for the two preparations. Their relative reconstituted transport activities and cytochalasin B binding activities were equal, indicating that the two were reconstituted with similar efficiencies. The decrease in glucose transport in erythrocytes produced by ATP depletion and the stimulation produced by resealing with ATP (Jacquez, J.A. (1983) Biochim. Biophys. Acta 727, 367-378) were confirmed. However, no difference was observed in reconstituted transport activity using ghosts resealed with or without ATP, indicating that ATP produces indirect effects rather than modifications of the transporter.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.