Inhibition of internalization of glucose transporters and IGF-II receptors. Mechanism of action of MHC class I-derived peptides which augment the insulin response in rat adipose cells

Peptides from the alpha 1 domain of the major histocompatibility complex class I antigen (MHC class I), e.g. Dk-(61-85) and Dk-(62-85), have been shown previously to augment glucose uptake in insulin-stimulated cells and to inhibit insulin receptor internalization (Stagsted, J., Reaven, G. M., Hansen, T., Goldstein, A., and Olsson, L. (1990) Cell 62, 297-307). We now report that these peptides inhibit by 80-100% the internalization of glucose transporters (GLUT4) and insulin-like growth factor II (IGF-II) receptors in insulin-stimulated cells and correspondingly double insulin-stimulated glucose transport activity and the number of GLUT4 and IGF-II receptors on the cell surface. In addition, the peptides enhance the apparent affinity about 3-fold of IGF-II binding to its receptor. It is concluded that the effects of the peptides on glucose transport and IGF-II binding are a consequence of the peptide-mediated inhibition of internalization of GLUT4 and IGF-II receptor. The active peptides are derived from the alpha 1 domain of a MHC class I molecule, suggesting that the latter is involved in regulation of internalization of cell surface integral membrane proteins such as the GLUT4 and IGF-II and insulin receptors.

The glucose transporter family: structure, function and tissue-specific expression

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Glut 4 content in the plasma membrane of rat skeletal muscle: comparative studies of the subcellular fractionation method and the exofacial photolabelling technique using ATB-BMPA

Employing subcellular membrane fractionation methods it has been shown that insulin induces a 2-fold increase in Glut 4 protein content in the plasma membrane of skeletal muscle from rats. Data based upon this technique are, however, impeded by poor plasma membrane recovery and cross-contamination with intracellular membrane vesicles. The present study was undertaken to compare the subcellular fractionation technique with the technique using [3H]ATB-BMPA exofacial photolabelling and immunoprecipitation of Glut 4 on soleus muscles from 3-week-old Wistar rats. Maximal insulin stimulation resulted in a 6-fold increase in 3-O-methylglucose uptake, and studies based on the subcellular fractionation method showed a 2-fold increase in Glut 4 content in the plasma membrane, whereas the exofacial photolabelling demonstrated a 6- to 7-fold rise in cell surface associated Glut 4 protein. Glucose transport activity was positively correlated with cell surface Glut 4 content as estimated by exofacial labelling.

IN CONCLUSION

(1) the increase in glucose uptake in muscle after insulin exposure is caused by an augmented concentration of Glut 4 protein on the cell surface membrane, (2) at maximal insulin stimulation (20 mU/ml) approximately 40% of the muscle cell content of Glut 4 is at the cell surface, and (3) the exofacial labelling technique is more sensitive than the subcellular fractionation technique in measuring the amount of glucose transporters on muscle cell surface.

Use of bismannose photolabel to elucidate insulin-regulated GLUT4 subcellular trafficking kinetics in rat adipose cells. Evidence that exocytosis is a critical site of hormone action

The subcellular trafficking of tracer-tagged GLUT4 between the plasma membranes and low-density microsomes of rat adipose cells has been studied. Cell-surface GLUT4 have been initially tracer-tagged in the insulin-stimulated state with the [3H]bismanose photolabel 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2- propylamine. The half-time for internalization of tracer-tagged GLUT4 when insulin is removed by collagenase treatment is similar to that observed for the decrease in immunodetectable GLUT4 in the plasma membranes and the decrease in glucose transport activity in the intact cells. In contrast, internalization of tracer-tagged GLUT4 also occurs when cells are maintained in the continuous presence of insulin even though the plasma membrane level of immunodetectable GLUT4 and glucose transport activity in the intact cells are unaltered. These data show, for the first time, that insulin has little, if any, effect on the rate constant for GLUT4 endocytosis, but instead, primarily increases the rate constant for exocytosis. Tracer-tagged GLUT4 that is returned to the low-density microsomes can be restimulated with fresh insulin to recycle to the plasma membranes and to a steady-state distribution level that is the same as that observed in cells that are maintained in the continuous presence of insulin. These data suggest that the cells' entire complement of GLUT4 is involved in the recycling process. Following insulin stimulation of adipose cells initially in the basal state, the increase in immunodetectable GLUT4 in the plasma membranes precedes the increase in accessibility of GLUT4 to exofacial 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2- propylamine photolabeling, and this in turn precedes the increase in cellular glucose transport activity. Such time course data suggest that there may be plasma membrane intermediate states in the GLUT4 trafficking pathway. The kinetic properties of GLUT4 translocation and its recycling have been interpreted in terms of a subcellular trafficking model that identifies exocytosis, possibly involving-hypothetical "docking" and "fusion" steps, as the critical site of hormone action.

Metformin blocks downregulation of cell surface GLUT4 caused by chronic insulin treatment of rat adipocytes

Large decreases in insulin-responsive glucose transport occur in rat adipocytes maintained in culture for 24 h in the continuous presence of insulin. After 24 h in culture, an acute treatment with insulin increased 3-O-methyl-D-glucose transport by only approximately fivefold. In chronically insulin-treated cells, the transport activity was more severely reduced. The transport activity was only approximately twofold higher than in basal cells. To attribute changes in transport to alterations in cell surface transporters, we labeled the cell surface GLUT4 and GLUT1 transporters with the impermeant photoaffinity label 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis(D-mannos -4-yloxy)-2- propylamine. Cell surface labeling was compared with the labeling obtained in digitonin-permeabilized cells where the normally impermeant reagent had access to the total cellular pool of transporters. Labeling showed that in basal cells the proportions of GLUT4 and GLUT1 at the cell surface were 20 and 22% of the total. After an acute treatment with insulin, the proportions of GLUT4 and GLUT1 at the cell surface were increased to 49 and 37% of the total, respectively. The chronic insulin treatment was associated with a very low proportion of GLUT4 (25% of the total) at the cell surface. The downregulation of GLUT4 observed after chronic insulin treatment was alleviated by metformin, and the proportion of GLUT4 at the cell surface was maintained at 60% of the total.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of D-fructose and its analogues by Trypanosoma brucei

Kinetic parameters for entry of D-fructose into Trypanosoma brucei brucei have been determined. The net uptake of D-fructose was found to be rapid and occurred at a rate which was comparable with that observed for uptake of D-glucose. The Km and Vmax were 3.91 +/- 1.58 mM and 69.1 +/- 7.2 nmol min-1 (mg protein)-1. D-Fructose was metabolized to pyruvate under aerobic conditions and to pyruvate and glycerol under anaerobic conditions in a manner similar to D-glucose. Comparisons of the kinetic parameters for D-fructose transport and metabolism indicated that uptake was rate limiting. Inhibition constants (Ki) for inhibition of 6-deoxy-D-glucose by D-fructose and D-fructose transport by 6-deoxy-D-glucose were consistent with the Km values for these two substrates. These interactions indicate that D-fructose and 6-deoxy-D-glucose share a single common transporter. 1,5-Anhydro-D-glucitol and 1,5 anhydro-D-mannitol (the fused pyranose ring analogues of D-glucose and D-mannose) have been found to interact well with the transporter, while L-sorbose (a D-fructose analogue with a pyranose ring) had only low affinity. However, 2,5-anhydro-D-mannitol (a fused furanose ring analogue of D-fructose) inhibited both 6-deoxy-D-glucose and D-fructose transport with a Ki of approx. 0.8 mM. The high affinity for 2,5-anhydro-D-mannitol (2-deoxy-D-fructofuranose) indicates that D-fructose is transported in the furanose ring form.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of GLUT4 and GLUT1 subcellular trafficking in basal and insulin-stimulated 3T3-L1 cells

The two glucose transporter isoforms GLUT4 and GLUT1 present in 3T3-L1 cells were labeled in the insulin-stimulated and basal states with the impermeant bis-mannose photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propylamine. The redistributions of these labeled transporters from the plasma membrane to the low density microsome membrane fraction were followed while cells were maintained at either insulin-stimulated or basal steady states. In both these steady states GLUT4 and GLUT1 were continuously recycled. Analysis of the time courses for tracer-tagged GLUT4 and GLUT1 redistribution showed that the endocytosis rate constants were only approximately 30% slower in the insulin-stimulated (0.08 and 0.093 min-1) compared with the basal (0.116 and 0.121 min-1) state. In the insulin-stimulated state, the rate constants for GLUT4 and GLUT1 exocytosis (0.086 and 0.096 min-1) were similar to those of endocytosis. In contrast, the exocytosis rate constants of GLUT4 and GLUT1 in the basal state were 0.01 and 0.035 min-1. We therefore conclude that the main effect of insulin is to increase GLUT4 and GLUT1 exocytosis rate constants by approximately 9- and 3-fold, respectively, and that the unique feature of the GLUT4 isoform is the very slow rate of exocytosis in the basal state.

Insulinomimetic effect on glucose transport by epidermal growth factor when combined with a major histocompatibility complex class I-derived peptide

Peptides derived from the alpha 1-region of the murine H-2Dk molecule enhance glucose uptake in rat adipose cells above the maximum obtained with insulin stimulation alone (Stagsted, J., Reaven, G. M., Hansen, T., Goldstein, A., and Olsson, L. (1990) Cell 62, 297-307). We now describe that epidermal growth factor (EGF) in combination with the same peptides, Dk-(61-85) and Dk-(62-85), stimulates cellular glucose uptake 5-7 times over the basal level, i.e. to 30-50% of the maximal insulin effect. EGF alone increased glucose uptake by only approximately 50% above basal and the peptide alone by 100% above basal. Maximal effect of EGF and peptide was reached in 10-20 min with 30 microM peptide (EC50 10-15 microM) and 50 nM EGF (EC50 1-2 nM). The effect of EGF and peptide on glucose uptake was additive to that of insulin and peptide until the maximal level attained with insulin and peptide was reached. The combined effect of EGF plus peptide on glucose transport was associated with a recruitment of GLUT4 molecules to the plasma membrane. However, the phosphatidylinositol (PI) kinase which is activated by insulin was not activated by EGF plus peptide. Thus, the effect of EGF plus peptide on glucose uptake seems independent of the activity status of the insulin receptor. 125I-Labeled EGF bound specifically to rat adipose cells with an apparent affinity of approximately 2 nM and Bmax approximately 5 x 10(3). However, the major histocompatibility complex (MHC) peptides did not affect EGF-stimulated internalization of EGF receptor, in contrast to their effect on the insulin receptors. Transforming growth factor alpha had an effect similar to EGF on glucose uptake. Three other peptides derived from other parts of murine MHC class I had no effect on glucose uptake in combination with EGF. Thus, EGF in combination with certain MHC class I-derived peptides is insulinomimetic concerning glucose transport and this effect is independent of the insulin receptor activity.

Cell surface accessibility of GLUT4 glucose transporters in insulin-stimulated rat adipose cells. Modulation by isoprenaline and adenosine

Insulin-stimulated glucose transport activity in rat adipocytes is inhibited by isoprenaline and enhanced by adenosine. Both of these effects occur without corresponding changes in the subcellular distribution of the GLUT4 glucose transporter isoform. In this paper, we have utilized the impermeant, exofacial bis-mannose glucose transporter-specific photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propylamine (ATB-BMPA) [Clark & Holman (1990) Biochem. J. 269, 615-622], to examine the cell surface accessibility of GLUT4 glucose transporters under these conditions. Compared with cells treated with insulin alone, adenosine in the presence of insulin increased the accessibility of GLUT4 to the extracellular photolabel by approximately 25%, consistent with its enhancement of insulin-stimulated glucose transport activity; the plasma membrane concentration of GLUT4 as assessed by Western blotting was unchanged. Conversely, isoprenaline, in the absence of adenosine, promoted a time-dependent (t1/2 approximately 2 min) decrease in the accessibility of insulin-stimulated cell surface GLUT4 of > 50%, which directly correlated with the observed inhibition of transport activity; the plasma membrane concentration of GLUT4 decreased by 0-15%. Photolabelling the corresponding plasma membranes revealed that these alterations in the ability of the photolabel to bind to GLUT4 are transient, as the levels of both photolabel incorporation and plasma membrane glucose transport activity were consistent with the observed GLUT4 concentration. These data suggest that insulin-stimulated GLUT4 glucose transporters can exist in two distinct states within the adipocyte plasma membrane, one which is functional and accessible to extracellular substrate, and one which is non-functional and unable to bind extracellular substrate. These effects are only observed in the intact adipocyte and are not retained in plasma membranes isolated from these cells when analysed for their ability to transport glucose or bind photolabel.

Site-directed mutagenesis of GLUT1 in helix 7 residue 282 results in perturbation of exofacial ligand binding

The structure-function relationship of the HepG2/erythrocyte-type glucose transporter (GLUT1) has been studied by in vitro site-directed mutagenesis. Chinese hamster ovary clones in which glucose transporters were transfected were shown by Western blotting with a GLUT1 anti-COOH-terminal peptide antibody to have expression levels of Gln282----Leu, Asn288----Ile, and Asn317----Ile mutations that were comparable with the wild type. All three mutant GLUT1 clones had high 2-deoxy-D-glucose transport activity compared with a nontransfected clone, suggesting that these residues are not absolutely required for the transport function. We have examined the possibility that the inner and outer portions of the transport pathway are structurally separate by measuring the interaction of the mutant transporters with the inside site-specific ligand cytochalasin B and the outside site-specific ligand 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2- propylamine (ATB-BMPA). All three mutant GLUT1 clones showed high levels of cytochalasin B labeling, and the N288I and N317I mutants showed high levels of ATB-BMPA labeling. In contrast to the transport and cytochalasin B labeling results, the transmembrane helix 7 Gln282----Leu mutant was labeled by ATB-BMPA to a level that was only 5% of the level observed in the wild type. We have confirmed that this mutant was defective in the outer site by comparing the inhibition of wild-type and mutant 2-deoxy-D-glucose transport by the outside site-specific ligand 4,6-O-ethylidene-D-glucose. 4,6-O-Ethylidene-D-glucose inhibited wild-type transport with a Ki of approximately 12 mM, but this was increased to greater than 120 mM in the Gln282----Leu mutant. Thus, of the 3 residues mutated in this study, only glutamine 282 substitution causes a major perturbation in function, and this is a specific and striking reduction in the affinity for the outside site-specific ligands ATB-BMPA and 4,6-O-ethylidene-D-glucose.

Photolabelling of the liver-type glucose-transporter isoform GLUT2 with an azitrifluoroethylbenzoyl-substituted bis-D-mannose

The bis-D-mannose photolabel ATB-BMPA (2-N-[4-(1-azi-2,2,2- trifluoroethyl)benzoyl]-1,3-bis-(D-mannos-4-yloxy) propyl-2-amine) has been used to radiolabel the glucose transporter present in liver plasma membranes. The labelling was inhibited by 4,6-O-ethylidene-D-glucose. Approx. 7% of the liver plasma-membrane protein that was photolabelled in a 4,6-O-ethylidene-D-glucose-inhibitable manner was specifically immunoprecipitated by either an anti-(GLUT2 C-terminal peptide) antibody or by an anti-(GLUT2 exofacial-loop peptide) antibody. After correction for non-specific labelling and precipitation, the ratio of immunoprecipitable GLUT2 to GLUT1 was approximately 5:1, suggesting that GLUT1 was not a major component of liver plasma membranes. The low levels of immunoprecipitation of the photolabelled transporter may be due to low antibody affinity for GLUT2 or may indicate that the photolabelling reagent has labelled another glucose-transporter-like protein. The hexose-transport inhibitors phloretin, cytochalasin B and 4,6-O-ethylidene-D-glucose all inhibited the photolabelling by ATB-BMPA of immunoprecipitable GLUT2. D-Glucose inhibited approx. 57% of the ATB-BMPA labelling of GLUT2. D-Fructose also inhibited the GLUT2 labelling confirming that it is a substrate for GLUT2 [Gould, Thomas, Jess & Bell (1991) Biochemistry 30, 5139-5145]. From photolabel displacement by a range of concentrations of non-labelled ATB-BMPA, the affinity constant (Kd) of ATB-BMPA was found to be 250 +/- 78 microM, whereas the Bmax. (total number of binding sites) value was 2.1 +/- 0.29 pmol of GLUT2/mg of membrane protein. Since GLUT1, GLUT4 and GLUT2 have approximately equal affinities for the external ligand ATB-BMPA, but have widely varying affinities for equilibrated and transported substrates, it is suggested that the isoforms may differ in their ability to bind hexoses at the internal site.

Development of an intracellular pool of glucose transporters in 3T3-L1 cells

The membrane-impermeant bis-mannose photolabel 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2- propylamine (ATB-BMPA) has been used to study the development of an intracellular pool of glucose transporters in 3T3-L1 cells. The subcellular distributions of the transporter isoforms GLUT1 and GLUT4 were determined by comparing the labeling obtained in cells in which the impermeant reagent only had access to the cell surface and the labeling obtained in digitonin-permeabilized cells. ATB-BMPA labeling showed that only GLUT1 was present in preconfluent fibroblasts and that most of the transporters were distributed to the cell surface. In preconfluent fibroblasts, the 2-deoxy-D-glucose transport activity was approximately 5 times higher than in confluent fibroblasts. ATB-BMPA labeling showed that the decrease in transport as cells reached confluence was associated with a decrease in the proportion of GLUT1 distributed to the cell surface. The sequestration of these transporters was associated with the development of an insulin-responsive transport activity which increased by approximately 2.5-fold compared with unstimulated confluent cells. ATB-BMPA labeling showed that insulin stimulation resulted in an approximately 2-fold increase in surface GLUT1 so that about one-half of the available transporters became recruited to the cell surface. Measurements of the changes in the distribution of both GLUT1 and GLUT4 throughout the differentiation of confluent fibroblasts into adipocytes showed that both transporters were sequestered in parallel. Basal levels of transport and photolabeling remained low throughout the differentiation period when the total pool of transporters (GLUT1 plus GLUT4) was increased by approximately 5-fold. These results suggest that the sequestration process was present before new transporters were synthesized. Thus, the sequestration mechanism develops in confluent growth-arrested fibroblasts although the capacity to sequester additional transporters may increase as differentiation proceeds.

Kinetic resolution of the separate GLUT1 and GLUT4 glucose transport activities in 3T3-L1 cells

A bis-mannose-photolabel-displacement method has been developed for resolving the separate kinetic properties of the glucose transporters GLUT1 and GLUT4, which are both present in 3T3-L1 cells. We have quantified the cell-surface transporter abundance (Bmax.) for the two isoforms by displacing radiolabelled 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis-(D- mannos-4-yloxy)-2-propylamine (ATB-BMPA) by non-labelled ATB-BMPA. In cells acutely treated with insulin, the GLUT1 Bmax. was 0.19 microM and the GLUT4 Bmax. was 0.17 microM. In cells which were chronically treated with insulin, the GLUT1 Bmax. was increased by approximately 4-fold to 0.7 microM, whereas the GLUT4 was decreased by approximately 50% (Bmax. = 0.1 microM). However, this large increase in total concentrations of cell-surface transporters (the sum of GLUT1 and GLUT4 concentrations) was not reflected in a large increase in 3-O-methyl-D-glucose transport, suggesting that GLUT1 makes a smaller contribution to transport than does GLUT4. In acutely insulin-treated cells at 37 degrees C, the apparent kinetic parameters for 3-O-methyl-D-glucose transport were Vapp.max. = 0.52 mM.s-1 and Kapp.m = 12.3 mM. In chronically insulin-treated cells the Vapp.max. = 1.24 mM.s-1 and Kapp.m = 23.0 mM. We have measured the displacement of ATB-BMPA by different concentrations of 3-O-methyl-D-glucose to resolve the separate affinity constants of GLUT1 and GLUT4 for this transported ligand. In acute- and chronic-insulin-treated cells the GLUT1 Km for 3-O-methyl-D-glucose was approximately 20 mM, and the GLUT4 Km for 3-O-methyl-D-glucose was approximately 7 mM. An analysis of these data and the 3-O-methyl-D-glucose transport rates was carried out to calculate transport capacity (TK values) for the two isoforms at 37 degrees C. In acute- and chronic-insulin-treated cells the TK values were 0.36 x 10(4) mM-1.min-1 for GLUT1 and 1.13 x 10(4) mM-1.min-1 for GLUT4. Thus GLUT1 has an approximately 3-fold lower transport capacity than GLUT4 at low concentrations of transported sugar. The lower GLUT1 transport capacity was shown to be mainly due to the high Km of GLUT1. The calculated turnover numbers were 7.2 x 10(4) min-1 for GLUT1 and 7.9 x 10(4) min-1 for GLUT4.

Trafficking of glucose transporters in 3T3-L1 cells. Inhibition of trafficking by phenylarsine oxide implicates a slow dissociation of transporters from trafficking proteins

We have compared the rates of insulin stimulation of cell-surface availability of glucose-transporter isoforms (GLUT1 and GLUT4) and the stimulation of 2-deoxy-D-glucose transport in 3T3-L1 cells. The levels of cell-surface transporters have been assessed by using the bismannose compound 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis(D-mannos -4-yloxy) propyl-2-amine (ATB-BMPA). At 27 degrees C the half-times for the appearance of GLUT1 and GLUT4 at the cell surface were 5.7 and 5.4 min respectively and were slightly shorter than that for the observed stimulation of transport activity (t 1/2 8.6 min). This lag may be due to a slow dissociation of surface transporters from trafficking proteins responsible for translocation. When fully-insulin-stimulated cells were subjected to a low-pH washing procedure to remove insulin at 37 degrees C, the cell-surface levels of GLUT1 and GLUT4 decreased, with half-times of 9.2 and 6.8 min respectively. These times correlated well with decrease in 2-deoxy-D-glucose transport activity that occurred during this washing procedure (t1/2 6.5 min). When fully-insulin-stimulated cells were treated with phenylarsine oxide (PAO), a similar decrease in transport activity occurred (t1/2 9.8 min). However, surface labelling showed that this corresponded with a decrease in GLUT4 only (t1/2 7.8 min). The cell-surface level of GLUT1 remained high throughout the PAO treatment. Light-microsome membranes were isolated from cells which had been cell-surface-labelled with ATB-BMPA. Internalization of both transporter isoforms to this pool occurred when cells were maintained in the presence of insulin for 60 min. In contrast with the surface-labelling results, we have shown that the transfer to the light-microsome pool of both transporters occurred in cells treated with insulin and PAO. These results suggest that both transporters are recycled by fluid-phase endocytosis and exocytosis. PAO may inhibit this recycling at a stage which involves the re-emergence of internalized transporters at the plasma membrane. The GLUT1 transporters that are recycled to the surface in insulin- and PAO-treated cells appear to have low transport activity. This may be because of a failure to dissociate fully from trafficking proteins at the cell surface. GLUT4 transporters appear to have a greater tendency to remain internalized if the normal mechanisms that commit transporters to the cell surface, such as dissociation from trafficking proteins, are uncoupled.

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)

Chronic treatment with insulin selectively down-regulates cell-surface GLUT4 glucose transporters in 3T3-L1 adipocytes

A new method for photoaffinity labeling of glucose transporters has been used to compare the effects of glucose-starvation, acute-insulin, and chronic-insulin treatments on the cell-surface glucose transporters in 3T3-L1 adipocytes. Starvation alone increased the cell-surface levels of GLUT1 and GLUT4 by approximately 4- and approximately 2-fold, respectively. As shown by Calderhead, D, M., Kitagawa, K., Tanner, L.T., Holman, G.D., and Lienhard, G.E. (1990) J. Biol. Chem. 265, 13800-13808) acute-insulin treatment increased cell-surface GLUT1 and GLUT4 by approximately 5- and approximately 15-fold respectively. In contrast to this, chronic-insulin treatment gave a further 3-4-fold increase in both cell-surface and total cellular GLUT1, but availability of GLUT4 at the cell-surface was down-regulated to half the level found in the acute treatment but with no change in the total cellular level. This effect occurred in starved and non-starved cells and suggests that starvation, acute-insulin, and chronic-insulin treatments regulate glucose transporter availability through independent mechanisms. The down-regulation of GLUT4 reached a maximally reduced cell-surface level in 6 h while the rise in GLUT1 reached a maximum after 24-48 h. The rise in GLUT1 appeared to compensate for the decline in cell-surface GLUT4 as glucose transport activity was further increased during the long term treatment with insulin. The down-regulation of GLUT4 due to the chronic-insulin treatment is associated with a marked resistance of the cells to restimulate glucose transport and particularly to recruit further GLUT4 to the cell-surface following an additional insulin treatment. The defect appears to be in the signaling mechanism that is responsible for translocation.

Phorbol ester only partially mimics the effects of insulin on glucose transport and glucose-transporter distribution in 3T3-L1 adipocytes

We examined the effects of the phorbol ester phorbol 12-myristate 13-acetate (PMA) on the rate of hexose transport into 3T3-L1 adipocytes. Exposure of adipocytes to PMA (1 microM) for 60 min results in a 1.7-2.5-fold increase in the rate of hexose transport. This effect was mediated by translocation of two isoforms of glucose transporters to the plasma membrane, as determined by labelling in situ, photoaffinity labelling with a membrane-impermeant glucose analogue, and by immunoblotting of subcellular fractions. The PMA-induced stimulation of both transport and transporter translocation was substantially less than that induced by insulin in this cell line; the PMA-induced increase in plasma-membrane GLUT 1 and GLUT 4 transporter isoforms was only about 40% and 10% respectively of that induced by insulin. We suggest that the stimulation of transport by insulin and PMA occurs via different mechanisms, which is manifested by the ability of insulin to induce a much greater increase in the plasma-membrane content of GLUT 4 compared with the phorbol ester.

Cell surface labeling of glucose transporter isoform GLUT4 by bis-mannose photolabel. Correlation with stimulation of glucose transport in rat adipose cells by insulin and phorbol ester

A new impermeant photoaffinity label has been used for identifying cell surface glucose transporters in isolated rat adipose cells. This compound is 2-N-4(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4- yloxy)-2- propylamine. We have used this reagent in combination with immunoprecipitation by specific antibodies against the GLUT4 and GLUT1 glucose transporter isoforms to estimate the relative abundance of these two transporters on the surface of the intact adipose cell following stimulation by insulin and phorbol 12-myristate 13-acetate (PMA). In the basal state, GLUT4 and GLUT1 are both present at the cell surface but GLUT4 is more abundant than GLUT1. In response to insulin, GLUT4 increases 15-20-fold and GLUT1 increases approximately 5-fold while 3-O-methyl-D-glucose transport is stimulated 20-30-fold. By contrast, PMA only induces a approximately 4-fold increase in GLUT4 while GLUT1 increases approximately 5-fold to the same level as seen with insulin. In addition, PMA stimulates 3-O-methyl-D-glucose transport approximately 3-fold to only 13% of the insulin-stimulated state. Thus GLUT4 is the major glucose transporter isoform under all conditions, and it is selectively and markedly enriched in response to insulin but not PMA which increases GLUT1 and GLUT4 equally. Furthermore, stimulation of glucose transport activity correlates closely with the appearance of GLUT4 on the cell surface in response to both insulin and PMA but does not correlate with the sum of GLUT1 and GLUT4 appearance. These results suggest that GLUT4 may be inherently more active than GLUT1 due to a higher TK (turnover/Km).

Insulin regulation of the two glucose transporters in 3T3-L1 adipocytes

The amounts of the brain type and muscle type glucose transporters (designated Glut 1 and 4, respectively) in 3T3-L1 adipocytes have been determined by quantitative immunoblotting with antibodies against their carboxyl-terminal peptides. There are about 950,000 and 280,000 copies of Glut 1 and 4, respectively, per cell. Insulin caused the translocation of both types of transporters from an intracellular location to the plasma membrane. The insulin-elicited increase in cell surface transporters was assessed by labeling the surface transporters with a newly developed, membrane-impermeant, photoaffinity labeling reagent for glucose transporters. The increases in Glut 1 and 4 averaged 6.5- and 17-fold, respectively, whereas there was a 21-fold in hexose transport. These results indicate that the translocation of Glut 4 could largely account for the insulin effect on transport rate, but only if the intrinsic activity of Glut 4 is much higher than that of Glut 1. The two transporters are colocalized intracellularly: vesicles (average diameter 72 nm) isolated from the intracellular membranes by immunoadsorption with antibodies against Glut 1 contained 95% of the Glut 4 and, conversely, vesicles isolated with antibodies against Glut 4 contained 85% of the Glut 1.

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.

Specificity and kinetics of hexose transport in Trypanosoma brucei

Transport of 6-deoxy-D-glucose was studied in Trypanosoma brucei in order to characterise the kinetics of hexose transport in this organism using a nonphosphorylated sugar. Kinetic parameters for efflux and entry, measured using zero-trans and equilibrium exchange protocols, indicate that the transporter is probably kinetically symmetrical. Comparison of the kinetic constants of D-glucose metabolism with those for 6-deoxy-D-glucose transport shows that transport across the plasma membrane is likely to be the rate-limiting step of glucose utilisation. The transport rate is nevertheless very fast and 6-deoxy-D-glucose, at concentrations below Km, enters the cells with a half filling time of less than 2 s at 20 degrees C. Thus the high metabolic capacity of these organisms is matched by a high transport rate. The structural requirements for the trypanosome hexose transporter were explored by measuring inhibition constants (Ki) for a range of D-glucose analogues including fluoro and deoxy sugars as well as epimeric hexoses. The relative affinities shown by these analogues indicated H-bonds from the carrier to the C-3, C-4 and C-5 hydroxyl oxygens and from the C-1 and C-3 hydroxyl hydrogens to the binding site. Hydrophobic interactions are likely at the C-2 and C-6 regions of the glucose molecule. Spatial constraints appear to occur around C-4 indicating that the transport site at this position is not freely open to the external solution as is the case with the mammalian hexose transporter. However, the trypanosome transporter appears to accept D-fructose but the common mammalian (erythrocyte type) hexose transporter does not.

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

The benzophenone derivative of 1,3-bis(D-mannos-4-yloxy)-2-propylamine (BB-BMPA) has been tested as an exofacial photoaffinity label for the sugar transport systems of human erythrocytes and rat adipocytes. The half-maximal inhibition constants for the reagent are 971 microM in erythrocytes and 536 microM in basal and 254 microM in insulin-treated adipocytes. The photolabelling of erythrocyte membranes is very specific for the 50 kDa transporter peptide and is completely displaced by D-glucose. The exofacial photoaffinity labelling of adipocytes also shows labelling of a 50 kDa transporter peptide, which is displaced by cytochalasin B, but extensive nonspecific labelling of a 75 kDa plasma membrane peptide occurs. The transporter is labelled in insulin-treated cells but not in basal cells which indicates that this in situ labelling technique selectively reveals only those transporters that visit and are active in the plasma membrane during the labelling period. This also indicates that in basal cells transporters do not turn over rapidly. Subcellular redistribution of transporters after the labelling period has been studied. Following incubation and washing at 37 degrees C in the presence of insulin, 30% of the transporters photolabelled at the plasma membrane are internalised and are found in the light microsome fraction of the cell. The proportion of transporter that is observed to be internalised is much greater than can be accounted for by a contamination of the light microsome fraction by plasma membrane. The labelled 50 kDa transporter peptide in the light microsomes is enriched when compared with the carry-over of the 75 kDa nonspecifically labelled plasma membrane peptide. Thus we have obtained direct evidence for transporter translocation.

Binding of cytochalasin B to trypsin and thermolysin fragments of the human erythrocyte hexose transporter

The cleavage of the human erythrocyte hexose transporter by the proteinases trypsin and thermolysin has been studied. When red cell membranes are treated with trypsin, washed and then photolabelled with cytochalasin B, a labelled peak at 18 kDa is obtained. This labelling of the cleaved transporter is D-glucose inhibitable. This probably indicates that the residual 36 kDa portion of the transporter is not required for binding of ligands. Extensive cleavage of the transporter with low concentrations of thermolysin only occurs when transporter is prelabelled with cytochalasin B. This indicates that covalently bound cytochalasin B can cause a conformational change which exposes the thermolysin cleavage site.