Reassessment of the translocation hypothesis by kinetic studies on hexose transport in isolated rat adipocytes

The effect of insulin and factors which have insulin-like activity on the kinetic parameters of 3-O-methyl-D-glucose (MeGlc) transport in rat adipocytes were assessed. Carrier-mediated uptake of MeGlc was estimated by the difference in the amounts of [14C]MeGlc and L-[3H]glucose taken up in cells under equilibrium exchange conditions at 37 degrees C. The Km and Vmax values in basal cells were 17.4 mM and 0.24 nmol/10(6) cells/s, respectively. Removal of endogenous adenosine by adenosine deaminase resulted in a 26% decrease in the basal rate due to a slight increase in the Km (19.6 mM) and a decrease in the Vmax value (0.20 nmol/10(6) cells/s). The maximum concentration (10 nM) of insulin decreased the Km to approximately one-half of the basal (7.1 mM) concomitant with an 8.5-fold increase in the Vmax value (2.04 nmol/10(6) cells/s). Submaximal concentrations (50 and 150 pM) of insulin, N6-phenylisopropyladenosine (1 microM), mechanical agitation of cells by centrifugal force (160 x g), low temperature (15 degrees C), 12-O-tetradecanoylphorbol-13-acetate (1 microM), and hydrogen peroxide (10 mM) all decreased the basal Km value to a range of 13.5-7.3 mM, concomitant with a 1.7-7.4-fold increase in the Vmax. A possible explanation for the alterations in the kinetic parameters may be that insulin and other factors cause the translocation of the mobile low-Km glucose transporters from an intracellular site to the cell surface, where the stationary high-Km transporters are located. Thus, when the Km and Vmax values of the hypothetical high-Km transporters were assumed to be 20 mM and 0.20 nmol/10(6) cells/s, respectively, and the Km of the low-Km transporters was assumed to be 7 mM, the theoretical Km decreased from 20 to 7.5 mM as the Vmax of the low-Km transporters increased from near 0 to 2.0 nmol/10(6) cells/s. The relation between empirical Km and Vmax values as affected by several agents and conditions followed closely the relation predicted by the above two-transporter model.     

Reassessment of insulin effects on the Vmax and Km values of hexose transport in isolated rat epididymal adipocytes

Effects of insulin on the kinetic parameters of hexose transport in rat epididymal adipocytes were re-examined. The transport activity was assessed by measuring the rate of uptake of 3-O-[3H]methyl-D-glucose (MeGlc) under equilibrium exchange and zero-trans conditions. The incubation was carried out at 37 degrees C in an infant incubator. During the incubation, the cell suspension (25%, v/v, in a total volume of 48 microliter) was mechanically swirled at a rate of 600 rpm (r = 2 mm). The swirling facilitated the rapid uptake of MeGlc without stimulating the basal transport activity by "mechanical agitation". The basal and insulin-treated cells were incubated under identical conditions, except for the length of the incubation period. The incubation was terminated by the addition of 350 microliters of 1 mM phloretin, which inhibited transport in approximately 0.06 s. The time course of MeGlc uptake was consistent with the view that the process was a multiple-phase reaction. The initial phase of the reaction was completed when the intracellular distribution space of MeGlc was approximately 1% of the total cell volume. Insulin (10 nM) increased the Vmax value of MeGlc uptake 16-fold in equilibrium exchange experiments and 18-fold in zero-trans experiments. At the same time, the hormone decreased the Km value of MeGlc uptake from 11.7 to 5.4 mM in equilibrium exchange experiments and from 9.7 to 4.8 mM in zero-trans experiments. It is concluded that the major effect of insulin on MeGlc uptake is to increase the Vmax value, but the hormone has the additional effect of lowering the apparent Km value.     

Evidence for functionally distinct glucose transporters in basal and insulin-stimulated adipocytes

The activity and Km of glucose transport of rat adipocytes are quite variable in the basal state. This could be due to differing levels of highly saturable transport against a background of less saturable transport. Such heterogeneity could lead to differing conclusions as to the Km of basal cells compared to insulin-stimulated cells depending on the choice of substrate, the range of concentrations tested, and the rigor of data analysis. In the present work, we used a cell preparation which was stable and partially activated by constant agitation. We used a two-component model to fit the concentration dependence of D-glucose uptake. We defined two parallel pathways of glucose entry, a high-affinity/low-capacity pathway and a low-affinity/high-capacity pathway. Both pathways were stereospecific and were inhibited by cytochalasin B. The low-affinity pathway in basal cells had 97% of the total capacity (Vmax) with a high Km (greater than 50 mM). A second pathway had a very low Km (less than 1 mM) and only 3% of the total capacity, but contributed to 30-60% of glucose uptake at 8 mM glucose. In insulin-stimulated cells, a pathway with a Km of 4-5 mM dominated and contributed 85% of glucose transport. The low-affinity but not the very high affinity pathway persisted in stimulated cells, but its contribution was only 10-15% of transport at 8 mM glucose. These results suggest the presence of at least two functionally distinct transporters whose respective contributions can be characterized by nonlinear regression of data over a wide range of glucose concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of the ability of insulin to activate the glucose-transport system in rat adipocytes.

Isolated rat adipocytes were used to assess the mechanisms of the ability of insulin to accelerate glucose transport. Glucose transport was determined by measuring the initial rates of 2-deoxyglucose uptake, and at 24 degrees C insulin increased the Vmax. of transport from 7.3 +/- 1 to 23.1 +/- 2 nmol/min per 10(6) cells, but the Km value remained unchanged (2.5, cf. 2.4 mM). When the Vmax. of basal and insulin-stimulated transport was measured as a function of temperature (15-37 degrees C), parallel Arrhenius plots were obtained yielding equal activation energies of approx. 59kJ/mol. Since both processes have equal activation energies the data indicate that insulin increases Vmax. by increasing the number of available carriers rather than enhancing intrinsic activity of already functioning carriers. Since the ability of insulin to activate glucose transport did not decrease with temperature (whereas plasma-membrane fluidity declines), it is suggested that lateral diffusion of insulin receptors within the plasma-membrane bilayer is not a rat-determining step in insulin action.     

Qualitative and quantitative comparison of glucose transport activity and glucose transporter concentration in plasma membranes from basal and insulin-stimulated rat adipose cells.

Conditions are described which allow the isolation of rat adipose-cell plasma membranes retaining a large part of the stimulatory effect of insulin in intact cells. In these membranes, the magnitude of glucose-transport stimulation in response to insulin was compared with the concentration of transporters as measured with the cytochalasin-B-binding assay or by immunoblotting with an antiserum against the human erythrocyte glucose transporter. Further, the substrate- and temperature-dependencies of the basal and insulin-stimulated states were compared. Under carefully controlled homogenization conditions, insulin-treated adipose cells yielded plasma membranes with a glucose transport activity 10-15-fold higher than that in membranes from basal cells. Insulin increased the transport Vmax. (from 1,400 +/- 300 to 15,300 +/- 3,400 pmol/s per mg of protein; means +/- S.E.M.; assayed at 22 degrees C) without any significant change in Km (from 17.8 +/- 4.4 to 18.9 +/- 1.4 nM). Arrhenius plots of plasma-membrane transport exhibited a break at 21 degrees C, with a higher activation energy over the lower temperature range. The activation energy over the higher temperature range was significantly lower in membranes from basal than from insulin-stimulated cells [27.7 +/- 5.0 kJ/mol (6.6 +/- 1.2 kcal/mol) and 45.3 +/- 2.1 kJ/mol (10.8 +/- 0.5 kcal/mol) respectively], giving rise to a larger relative response to insulin when transport was assayed at 37 degrees C as compared with 22 degrees C. The stimulation of transport activity at 22 degrees C was fully accounted for by an increase in the concentration of transporters measured by cytochalasin B binding, if a 5% contamination of plasma membranes with low-density microsomes was assumed. However, this 10-fold stimulation of transport activity contrasted with an only 2-fold increase in transporter immunoreactivity in membranes from insulin-stimulated cells. These data suggest that, in addition to stimulating the translocation of glucose transporters to the plasma membrane, insulin appears to induce a structural or conformational change in the transporter, manifested in an altered activation energy for plasma-membrane transport and possibly in an altered immunoreactivity as assessed by Western blotting.     

Inhibition of insulin-stimulated glucose transport in rat adipocytes by nucleoside transport inhibitors

In isolated rat adipocytes, basal as well as insulin-stimulated 3-O-methylglucose transport was inhibited nearly completely (maximal inhibition: 95%) by the nucleoside transport inhibitors dipyridamole (IC50 = 5 microM), nitrobenzylthioguanosine (20 microM), nitrobenzylthioinosine (35 microM) and papaverine (130 microM). Transport kinetics in the presence of 10 microM dipyridamole revealed a significant increase in the transport Km value of 3-O-methylglucose (3.45 +/- 0.6 vs 2.36 +/- 0.29 mM in the controls) as well as a decrease in the Vmax value (4.84 +/- 0.95 vs 9.03 +/- 1.19 pmol/s per microliter lipid in the controls). Half-maximally inhibiting concentrations of dipyridamole were one order of magnitude higher than those inhibiting nucleoside (thymidine) uptake (0.48 microM). The inhibitory effect of dipyridamole (5 microM) reached its maximum within 30 s. The agent failed to affect insulin's half-maximally stimulating concentration (0.075 nM) indicating that it did not interfere with the mechanism by which insulin stimulates glucose transport. Further, dipyridamole fully suppressed the glucose-inhibitable cytochalasin B binding (IC50 = 1.65 +/- 0.05 microM). The data indicate that nucleoside transport inhibitors reduce glucose transport by a direct interaction with the transporter or a closely related protein. It is suggested that glucose and nucleoside transporters share structural, and possibly functional, features.     

Insulin and phorbol esters affect the maximum velocity rather than the half-saturation constant of 3-O-methylglucose transport in rat adipocytes

The kinetics of the equilibrium exchange flux of 3-O-methylglucose (MeGlc) were examined in isolated rat adipocytes using a recently described technique (Whitesell, R. R., and Abumrad, N. A. (1985) J. Biol. Chem. 260, 2894-2899) in which the cells, under basal conditions, were reported to exhibit a high Km (35 mM) that was reduced (to 3 mM) upon treatment with insulin. When this technique was employed in the present study, the Km observed in basal adipocytes was 6.4 +/- 0.4 mM; insulin treatment did not affect this parameter (6.3 +/- 0.5 mM), although it increased the maximum velocity (phi max) 21-fold (from 3.0 +/- 0.3 to 63.7 +/- 1.1 nmol X min-1 X microliter of intracellular water-1). The large discrepancy in the basal Km values observed in the previous (35 mM) and the present (6.4 mM) studies is shown to be associated with relatively minor differences in basal MeGlc flux; these minor differences may reflect insufficient mixing of labeled MeGlc in the flux measurements of the previous study. In addition, the active phorbol ester, 12-O-tetradecanoylphorbol 13-acetate, at a concentration of 0.3 microM, caused a 2.8-fold elevation of phi max, with no modulation of Km. These results indicate that phi max, not Km, is the major kinetic parameter of hexose transport affected by insulin and phorbol esters, leading to enhancement of hexose uptake by the isolated rat adipocyte.     

Protein synthesis inhibitors activate glucose transport without increasing plasma membrane glucose transporters in 3T3-L1 adipocytes

In this study, we tested the hypothesis that hexose transport regulation may involve proteins with relatively rapid turnover rates. 3T3-L1 adipocytes, which exhibit 10-fold increases in hexose transport rates within 30 min of the addition of 100 nM insulin, were utilized. Exposure of these cells to 300 microM anisomycin or 500 microM cycloheximide caused a maximal, 7-fold increase in 2-deoxyglucose transport rate after 4-8 h. The effects due to either insulin (0.5 h) or anisomycin (5 h) on the kinetics of zero-trans 3-O-methyl[14C]glucose transport were similar, resulting in 2.5-3-fold increases in apparent Vmax values (control Vmax = 1.6 +/- 0.3 x 10(-7) mmol/s/10(6) cells) coupled with approximately 2-fold decreases in apparent Km values (control Km = 23 +/- 3.3 mM). Insulin elicited the expected increases in plasma membrane levels of HepG2/erythrocyte (GLUT1) and muscle/adipocyte (GLUT4) transporters (1.6- and 2.8-fold, respectively) as determined by protein immunoblotting. In contrast, neither total cellular contents nor plasma membrane levels of these two transporter isoforms were increased when 3T3-L1 adipocytes were treated with either anisomycin or cycloheximide. 3-[125I]Iodo-4-azidophenethylamido-7-O-succinyldeacetylforskoli n labeling of glucose transporters in plasma membrane fractions of similarly treated cells was also unaffected by these agents. Thus, a striking discrepancy was observed between the marked increase in cellular hexose transport rates due to these protein synthesis inhibitors and the unaltered amounts of glucose transporter proteins in the plasma membrane fraction. These data indicate that short-term protein synthesis inhibition in 3T3-L1 adipocytes leads to large increases in the intrinsic catalytic activity of one or both of the GLUT1 and GLUT4 transporter isoforms.     

Cycloheximide decreases glucose transporters in rat adipocyte plasma membranes without affecting insulin-stimulated glucose transport.

This study examines the relationship between insulin-stimulated glucose transport and insulin-induced translocation of glucose transporters in isolated rat adipocytes. Adipose cells were incubated with or without cycloheximide, a potent inhibitor of protein synthesis, for 60 min and then for an additional 30 min with or without insulin. After the incubation we measured 3-O-methylglucose transport in the adipose cells, and subcellular membrane fractions were prepared. The numbers of glucose transporters in the various membrane fractions were determined by the cytochalasin B binding assay. Basal and insulin-stimulated 3-O-methylglucose uptakes were not affected by cycloheximide. Furthermore, cycloheximide affected neither Vmax. nor Km of insulin-stimulated 3-O-methylglucose transport. In contrast, the number of glucose transporters in plasma membranes derived from cells preincubated with cycloheximide and insulin was markedly decreased compared with those from cells incubated with insulin alone (10.5 +/- 0.8 and 22.2 +/- 1.8 pmol/mg of protein respectively; P less than 0.005). The number of glucose transporters in cells incubated with cycloheximide alone was not significantly different compared with control cells. SDS/polyacrylamide-gel-electrophoretic analysis of [3H]cytochalasin-B-photolabelled plasma-membrane fractions revealed that cycloheximide decreases the amount of labelled glucose transporters in insulin-stimulated membranes. However, the apparent molecular mass of the protein was not changed by cycloheximide treatment. The effect of cycloheximide on the two-dimensional electrophoretic profile of the glucose transporter in insulin-stimulated low-density microsomal membranes revealed a decrease in the pI-6.4 glucose-transporter isoform, whereas the insulin-translocatable isoform (pI 5.6) was decreased. Thus the observed discrepancy between insulin-stimulated glucose transport and insulin-induced translocation of glucose transporters strongly suggests that a still unknown protein-synthesis-dependent mechanism is involved in insulin activation of glucose transport.     

Action of insulin on the transport of 3-O-methylglucose in frog sartorius muscles

The initial rate of entry of 3-O-methyl-D-glucose into isolated frog sartorius muscles was measured at various concentrations of substrate, at 0, 9, 19, or 29 degrees C, after prior incubation at 19 degrees C with a maximally stimulating concentration of insulin. Control muscles were treated similarly, except for the omission of insulin. A saturable transport system provided for most of the entry of 3-O-methylglucose into muscle cells, but a small amount of penetration occurred by a nonsaturable route. The major effect of insulin was to produce a large increase in activity of the suturable system. The Vmax of entry increased, but there was no significant change in the apparent Km. The ratio of insulin-stimulated to basal Vmax was 10 when transport was measured at 29 degrees C but was 22 at 0 degrees C. These findings support the hypothesis that, although a large part of the effect of insulin on sugar transport can be accounted for by an increase in the number of functional transporters in the plasma membrane, there is a separate hormonal effect that permits a relatively greater activity of transporters at lower temperatures, compared with control rates. An additional effect of insulin was to produce a small but definite increase in the entry of sugar by the nonsaturable transport system.     

One transporter per vesicle: determination of the basis of the insulin effect on glucose transport

The basis for insulin stimulation of glucose transport in rat adipocytes has been investigated by determining the relative number of functional glucose transporters in the plasma and microsomal membranes from basal and insulin-treated cells. Each fraction was solubilized with cholate and then reconstituted into vesicles of about 500 A in diameter through removal of the cholate by dialysis. This procedure distributed the glucose transporters into the vesicles at a density of either one or none per vesicle. Consequently the fraction of the intravesicular volume that rapidly equilibrated with D-glucose provided an estimate of the relative number of functional transporters. By means of this one-transporter-per-vesicle method, it was found that insulin increased the number of transporters in the plasma membrane by a factor of 2.4 and decreased the number in the microsomes to 68% of the original value. These results provide independent evidence for the hypothesis that insulin causes the translocation of functional transporters from an intracellular location to the plasma membrane.     

Mechanism of insulin resistance in the post receptor events in sheep: 3-O-methylglucose transport in ovine adipocytes

A severe resistance to the stimulatory action of insulin on glucose metabolism has been shown in ruminant adipose tissue or isolated adipocytes as compared to that of rats. To elucidate the mechanism of insulin resistance in ruminants, we measured the stimulatory effect of insulin on 3-O-methylgulose transport and on intracellular glucose metabolism in isolated adipocytes from sheep and rats. At a glucose concentration (0.1 mM) where transport is thought to be rate-limiting for metabolism, lipogenesis from [U-14C]glucose by ovine adipocytes was markedly less than by rat adipocytes in both the basal state and at all insulin concentrations. The responsiveness to insulin assessed by percent increase above basal was reduced to about 15% of that in rat adipocytes, but the insulin sensitivity was similar, because the insulin concentration giving half-maximal stimulation, ED50, did not differ significantly between ovine and rat adipocytes. The maximal insulin-stimulated 3-O-methylglucose transport in ovine adipocytes per cell was less than 20% of that in rat adipocytes, with a significant lowering in basal rates of transport. However, when data was expressed per 3-O-methylglucose equilibrium space no significant differences were found between ovine and rat in the basal transport rates, but a lowered ability of insulin to stimulate glucose transport was still seen in ovine adipocytes. The dose-response curve for glucose transport was slightly shifted to the right in ovine adipocytes compared to rat adipocytes, indicating a small decrease in insulin sensitivity. The decrease in glucose transport was due to 60% reduction in the maximum velocity in the insulin--stimulated state, with no change in the Km.     

Insulin regulation of glucose metabolism in HT29 colonic adenocarcinoma cells: activation of glycolysis without augmentation of glucose transport

The effects of insulin on glucose transport and metabolism were examined in cultured HT29 human colonic adenocarcinoma cells. The presence of glucose transporters was verified by D-glucose displaceable [3H]cytochalasin B binding. The Kd and Bmax values from cytochalasin B binding studies were 190 +/- 30 nM and 8.4 +/- 1.4 pmol/mg protein, respectively. Glucose transport determined with 3-O-methylglucose showed saturable kinetics with a Km of 5.8 +/- 0.4 mM and a Vmax of 0.047 +/- 0.003 mumol/mg protein per min at 25 degrees C. Moreover, in HT29 cells, two classes of insulin binding sites were detected in radioligand binding experiments. Although insulin failed to stimulate glucose transport, it was found to activate glycolysis in HT29 cells. Glucose consumption increased from 0.33 +/- 0.03 mumol/mg protein per h to 0.49 +/- 0.05 mumol/mg protein per h and lactate production was augmented from 0.67 +/- 0.04 mumol/mg protein per h to 0.87 +/- 0.06 mumol/mg protein per h in response to 10(-7) to 10(-5) M insulin. Insulin also enhanced mannose metabolism. Apart from these two hexoses, HT29 cells exhibited a surprisingly narrow substrate specificity. With the possible exception of glyceraldehyde, little lactate was produced from alternative substrates, including adenosine, inosine, ribose, deoxyribose, dihydroxyacetone, galactose and fructose either with or without insulin. Despite its limited utilization by the glycolytic pathway, adenosine was readily salvaged for de novo synthesis of adenine nucleotides. These findings suggest that insulin directly influences substrate utilization through the glycolytic pathway in HT29 cells without activating the glucose transport pathway.     

Photoaffinity labeling of insulin-sensitive hexose transporters in intact rat adipocytes. Direct evidence that latent transporters become exposed to the extracellular space in response to insulin

Irradiation of intact rat adipocytes with high intensity ultraviolet light in the presence of 0.5 microM [3H] cytochalasin B results in the labeling of Mr 43,000 and 46,000 proteins that reside in the plasma membrane fraction. In contrast to the Mr 46,000 protein, the Mr 43,000 component is not observed in the microsome fraction and exhibits lower affinity for [3H]cytochalasin B. Photolabeling of the Mr 43,000 protein is inhibited by cytochalasin D, indicating it is not a hexose transporter component. The Mr 46,000 protein exhibits characteristics expected for the glucose transporter such that D-glucose or 3-O-methylglucose but not cytochalasin D inhibits its photolabeling with [3H] cytochalasin B. Furthermore, insulin addition to intact cells either prior to or after photoaffinity labeling of the Mr 46,000 protein causes a redistribution of this component from the low density microsomes to the plasma membrane fraction, as expected for the hexose transporter. Photolabeling of transporters in both the low density microsome and plasma membrane fractions is inhibited when intact cells are equilibrated with 50 mM ethylidene glucose prior to irradiation with [3H]cytochalasin B. Incubation of intact cells with 50 mM ethylidene glucose for 1 min at 15 degrees C leads to an intracellular concentration of only 2 mM. Under these conditions, the photoaffinity labeling in intact cells of hexose transporters that fractionate with the low density microsomes is unaffected, indicating these transporters are not exposed to the extracellular medium. In contrast, photolabeling in intact insulin-treated cells of hexose transporters that fractionate with the plasma membrane is inhibited under these incubation conditions. The results demonstrate that insulin action results in the exposure to the extracellular medium of previously sequestered hexose transporters.     

Proposed mechanism of insulin-resistant glucose transport in the isolated guinea pig adipocyte. Small intracellular pool of glucose transporters

A marked resistance to the stimulatory action of insulin on glucose metabolism has previously been shown in guinea pig, compared to rat, adipose tissue and isolated adipocytes. The mechanism of insulin resistance in isolated guinea pig adipocytes has, therefore, been examined by measuring 125I-insulin binding, the stimulatory effect of insulin on 3-0-methylglucose transport and on lipogenesis from [3-3H]glucose, the inhibitory effect of insulin on glucagon-stimulated glycerol release, and the translocation of glucose transporters in response to insulin. The translocation of glucose transporters was assessed by measuring the distribution of specific D-glucose-inhibitable [3H]cytochalasin B binding sites among the plasma, and high and low density microsomal membrane fractions prepared by differential centrifugation from basal and insulin-stimulated cells. At a glucose concentration (0.5 mM) where transport is thought to be rate-limiting for metabolism, insulin stimulates lipogenesis from 30 to 80 fmol/cell/90 min in guinea pig cells and from 25 to 380 fmol/cell/90 min in rat cells with half-maximal effects at approximately 100 pM in both cell types. Insulin similarly stimulates 3-O-methylglucose transport from 0.40 to 0.70 fmol/cell/min and from 0.24 to 3.60 fmol/cell/min in guinea pig and rat fat cells, respectively. Nevertheless, guinea pig cells bind more insulin per cell than rat cells, and insulin fully inhibits glucagon-stimulated glycerol release. In addition, the differences between guinea pig and rat cells in the stimulatory effect of insulin on lipogenesis and 3-O-methylglucose transport cannot be explained by the greater cell size of the former compared to the latter (0.18 and 0.09 micrograms of lipid/cell, respectively). However, the number of glucose transporters in the low density microsomal membrane fraction prepared from basal guinea pig cells is markedly reduced compared to that from rat fat cells (12 and 70 pmol/mg of membrane protein, respectively) and the translocation of intracellular glucose transporters to the plasma membrane fraction in response to insulin is correspondingly reduced. These results suggest that guinea pig adipocytes are markedly resistant to the stimulatory action of insulin on glucose transport and that this resistance is the consequence of a relative depletion in the number of intracellular glucose transporters.     

Insulin induces chloroquine-sensitive recycling of insulin-like growth factor II receptors but not of glucose transporters in rat adipocytes

Incubation of insulin-treated rat adipocytes with chloroquine, in a time- and concentration-dependent manner, was observed to inhibit the insulin-stimulated increase in insulin-like growth factor II (IGF-II) binding activity, whereas no significant change in IGF-II binding was observed in the absence of insulin. The incremental increase of insulin-stimulated IGF-II binding was inhibited 50% by 0.2 mM chloroquine within 15 min and was nearly completely abolished by 60 min. Interestingly, IGF-II binding was never observed to decrease below the binding value in cells without insulin treatment even when incubation was extended to 180 min. Scatchard analysis of IGF-II binding as well as the specific binding of an anti-IGF-II receptor antibody demonstrated that the loss of IGF-II binding in the insulin-stimulated chloroquine-treated adipocytes was due to a decrease in the number of cell-surface IGF-II receptors, whereas the total number of cellular IGF-II receptors was unaltered. The effect of chloroquine was observed to be reversible, temperature-dependent, and sensitive to the metabolic poison KCN. Furthermore, NH4Cl was also observed to inhibit insulin-stimulated increase in IGF-II binding. In contrast, chloroquine or NH4Cl did not inhibit the basal or insulin-stimulated glucose transport activity. Photoaffinity labeling of the glucose transporter with [3H]cytochalasin B also demonstrated that the basal and insulin-stimulated subcellular distribution of the glucose transporters was unaltered by chloroquine treatment. These results suggest that 1) insulin induces a constitutive, acidotropic agent-sensitive recycling of IGF-II receptor and 2) the glucose transporter and IGF-II receptor do not share the same insulin-regulated intracellular trafficking pathways.     

Dissociation of insulin-stimulated glucose transport from the translocation of glucose carriers in rat adipose cells

Cycloheximide, a potent inhibitor of protein synthesis, has been used to examine the relationship between recruitment of hexose carriers and the activation of glucose transport by insulin in rat adipocytes. Adipocytes were preincubated +/- cycloheximide for 90 min then +/- insulin for a further 30 min. We measured 3-O-methylglucose uptake in intact cells and in isolated plasma membrane vesicles. The concentration of glucose transporters in plasma membranes and low density microsomes was measured using a cytochalasin B binding assay. Cycloheximide had no affect on basal or insulin-stimulated 3-O-methylglucose uptake in intact cells or in plasma membrane vesicles. However, the number of glucose carriers in plasma membranes prepared from cells incubated with cycloheximide and insulin was markedly reduced compared to that from cells incubated with insulin alone (14 and 34 pmol/mg protein, respectively). Incubation of cells with cycloheximide alone did not change the concentration of glucose carriers in either plasma membranes or in low density microsomes compared to control cells. When isolated membranes were analyzed with an antiserum prepared against human erythrocyte glucose transporter, decreased cross-reactivity was observed in plasma membranes prepared from cycloheximide/insulin-treated cells compared to those from insulin cells. The present findings indicate that incubation of adipocytes with cycloheximide greatly reduces the number of hexose carriers in the plasma membrane of insulin-stimulated cells. Despite this reduction, insulin is still able to maximally stimulate glucose uptake. Thus, these data suggest an apparent dissociation between insulin stimulation of glucose transport activity and the recruitment of glucose carriers by the hormone.     

Characterization of a glucose transport system in Vibrio parahaemolyticus.

Cells of a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system)-negative mutant of Vibrio parahaemolyticus transport D-glucose in the presence of Na+. Maximum stimulation of D-glucose transport was observed at 40 mM NaCl, and Na+ could be replaced partially with Li+. Addition of D-glucose to the cell suspension under anaerobic conditions elicited Na+ uptake. Thus, we conclude that glucose is transported by a Na+/glucose symport mechanism. Calculated Vmax and Km values for the Na(+)-dependent D-glucose transport were 15 nmol/min/mg of protein and 0.57 mM, respectively, when NaCl was added at 40 mM. Na+ lowered the Km value without affecting the Vmax value. D-Glucose was the best substrate for this transport system, followed by galactose, alpha-D-fucose, and methyl-alpha-glucoside, judging from the inhibition pattern of the glucose transport. D-Glucose itself partly repressed the transport system when cells were grown in its presence.     

Cyclic AMP acutely stimulates translocation of the major insulin-regulatable glucose transporter GLUT4.

Facilitated glucose transport across plasma membranes is mediated by a family of transporters (GLUT1-GLUT5) that have different tissue distributions and Km values for transport. It has been shown that insulin stimulates glucose transport in fat and muscle tissues by causing the redistribution of one of these proteins (GLUT4) from inside the cell to the plasma membrane. Previous studies have shown that agents that change cAMP levels are able to modulate glucose transport in fat cells. The aim of this study was to investigate the mechanisms responsible for modulation of glucose transport by cAMP. 2-Deoxyglucose transport and insulin-regulatable glucose transporter (GLUT4) immunoreactivity in plasma and low density microsomal membranes were measured in adipocytes incubated for 30 min with insulin or dibutyryl-cAMP (Bt2cAMP). Low concentrations of Bt2cAMP (10 microM) increased 2-deoxyglucose uptake by translocating GLUT4 from low density microsomal membranes to the plasma membranes. Bt2cAMP at 1000 microM inhibited glucose transport below basal but further increased translocation of GLUT4. The effect of Bt2cAMP on translocation was additive to that of 7 nM insulin. We conclude that in rat adipocytes, Bt2cAMP acutely translocates GLUT4 but inhibits its activity to transport glucose.     

Insulin-mediated translocation of glucose transporters from intracellular membranes to plasma membranes: sole mechanism of stimulation of glucose transport in L6 muscle cells

Plasma membranes and light microsomes were isolated from fused L6 muscle cells. Pre-treatment of cells with insulin did not affect marker enzyme or protein distribution in isolated membranes. The number of glucose transporters in the isolated membranes was calculated from the D-glucose-protectable binding of [3H]cytochalasin B. Glucose transporter number was higher in plasma membranes and lower in intracellular membranes derived from insulin-treated cells than in the corresponding fractions from untreated cells. The net increase in glucose transporters in plasma membranes was identical to the net decrease in glucose transporters in light microsomes (2 pmol/1.23 x 10(8) cells). The fold increase in glucose transporter number/mg protein in plasma membranes (2-fold) was similar to the fold increase in glucose transport caused by insulin. This suggests that recruitment of glucose transporters from intracellular membranes to the plasma membrane is the major mechanism of stimulation of hexose transport in L6 muscle cells. This is the first report of isolation of the two insulin-sensitive membrane elements from a cell line, and the results indicate that, in contrast to rat adipocytes, there is not change in the intrinsic activity of the transporters in response to insulin.