Selective macrophage ascorbate deficiency suppresses early atherosclerosis

To test whether severe ascorbic acid deficiency in macrophages affects progression of early atherosclerosis, we used fetal liver cell transplantation to generate atherosclerosis-prone apolipoprotein E-deficient (apoE(-/-)) mice that selectively lacked the ascorbate transporter (SVCT2) in hematopoietic cells, including macrophages. After 13 weeks of chow diet, apoE(-/-) mice lacking the SVCT2 in macrophages had surprisingly less aortic atherosclerosis, decreased lesion macrophage numbers, and increased macrophage apoptosis compared to control-transplanted mice. Serum lipid levels were similar in both groups. Peritoneal macrophages lacking the SVCT2 had undetectable ascorbate; increased susceptibility to H(2)O(2)-induced mitochondrial dysfunction and apoptosis; decreased expression of genes for COX-2, IL1β, and IL6; and decreased lipopolysaccharide-stimulated NF-κB and antiapoptotic gene expression. These changes were associated with decreased expression of both the receptor for advanced glycation end products and HIF-1α, either or both of which could have been the proximal cause of decreased macrophage activation and apoptosis in ascorbate-deficient macrophages.

Compartmentalization of transport and phosphorylation of glucose in a hepatoma cell line

The first steps of glucose metabolism are carried out by members of the families of GLUTs (glucose transporters) and HKs (hexokinases). Previous experiments using the inhibitor of glucose transport, CB (cytochalasin B), revealed that compartmentalization of GLUTs and HKs is a major factor in the control of glucose uptake in L6 myotubes [Whitesell, Ardehali, Printz, Beechem, Knobel, Piston, Granner, Van Der Meer, Perriott and May (2003) Biochem. J. 370, 47-56]. In the present paper, we evaluate compartmentalization of GLUTs and HKs in a hepatoma cell line, H4IIE, which is characterized by excess GLUT activity, HKI in a particulate and a cytosolic fraction, and insignificant G6Pase (glucose-6-phosphatase) activity. The measured activity of glucose transport exceeded the rate of phosphorylation approx. 30-fold. Treatment with 25 microM CB (K(i) approximately 3 microM in H4IIE cells) paradoxically increased the excess of GLUTs over phosphorylation (GLUTs are inhibited 80%, while phosphorylation is inhibited 98%). The global relationships of the data could be reconciled most simply by a two-compartment model. In this model, phosphorylation of glucose is carried out by a subset of HK molecules supplied by a subset of GLUTs that are more sensitive to CB than the other GLUTs. The agent, DCC (dicyclohexylcarbodi-imide) caused HKI to translocate from the particulate compartment to the cytosolic compartment and potently inhibited glucose phosphorylation. The particulate compartment may represent the mitochondria, to which the more CB-sensitive GLUTs may control the transport of glucose.

Generation of oxidant stress in cultured endothelial cells by methylene blue: protective effects of glucose and ascorbic acid

The thiazine dye methylene blue has long been used to stimulate cellular redox metabolism. To determine the extent to which it also generates oxidant stress in cells, its effects in cultured human-derived endothelial cells were studied. As expected, low concentrations of the dye (2-20 microM) activated the pentose phosphate pathway and oxidized both NADPH and NADH. Methylene blue enhanced extracellular ferricyanide reduction, indicating that the reduced form of the dye was present outside the cells. This reduction was greater when ferricyanide was added just before rather than 15 min after methylene blue, confirming that the dye is at least initially reduced at the cell surface. In the absence of glucose, methylene blue at concentrations above 5 microM increased intracellular oxidant stress, as manifest by oxidation of dihydrofluorescein and cellular GSH. Inclusion of glucose protected against these effects. In cells that had been loaded with ascorbate, the dye caused progressive oxidation of ascorbate, even in the presence of D-glucose. Loading cells with ascorbate also partially prevented oxidation of dihydrofluorescein by methylene blue. These results suggest that concentrations of the dye above 5 microM generated intracellular reactive oxygen species that were scavenged by ascorbate and GSH. Further, although D-glucose enhanced reduction of methylene blue, it ameliorated the oxidant stress generated by the dye.

Control of glucose phosphorylation in L6 myotubes by compartmentalization, hexokinase, and glucose transport

In muscle, insulin enhances influx of glucose and its conversion to glucose 6-phosphate (G6P) by hexokinase (HK). While effects of insulin on glucose transport have been demonstrated, its effect on the activity of HK of cells has not. In L6 myotubes treated for 24 h with insulin there was increased expression of the HK isoform, HKII, and increased glucose phosphorylation without a concomitant increase in glucose transport, indirectly suggesting that phosphorylation of glucose was a target of insulin action [Osawa, Printz, Whitesell and Granner (1995) Diabetes 44, 1426-1432]. In the present work the same treatment led to a 2-fold rise in G6P, suggesting that transport and/or HK were important targets of insulin action. We used a method to identify the site of rate control involving the specificity of phosphorylation towards 2-deoxy-[1-14C]glucose and D-[2-3H]glucose. Glucose transport does not greatly discriminate between these two tracers while HK shows increased specificity for glucose. Specificity of the glucose phosphorylation of the cells increased with addition of insulin and when extracellular glucose was raised. Specificity was reduced at low glucose concentrations or when the inhibitor of transport, cytochalasin B, was added. We conclude that transport and HK share nearly equal control over glucose phosphorylation in these cells. A computer program was used to test models for compatibility with the different types of experiments. The predicted intracellular glucose and transport rates associated with phosphorylation activity were lower than their measured values for the whole cell. In the most likely model, 15+/-4% of the glucose transporters serve a proportionate volume of the cytoplasm. Insulin activation of glucose phosphorylation might then result from stimulation of these transporters together with HK recruitment or relief from inhibition by G6P.

Uptake, recycling, and antioxidant actions of alpha-lipoic acid in endothelial cells

Alpha-lipoic acid, which becomes a powerful antioxidant in its reduced form, has been suggested as a dietary supplement to treat diseases associated with excessive oxidant stress. Because the vascular endothelium is dysfunctional in many of these conditions, we studied the uptake, reduction, and antioxidant effects of alpha-lipoic acid in cultured human endothelial cells (EA.hy926). Using a new assay for dihydrolipoic acid, we found that EA.hy926 cells rapidly take up and reduce alpha-lipoic acid to dihydrolipoic acid, most of which is released into the incubation medium. Nonetheless, the cells maintain dihydrolipoic acid following overnight culture, probably by recycling it from alpha-lipoic acid. Acute reduction of alpha-lipoic acid activates the pentose phosphate cycle and consumes nicotinamide adenine dinucleotide phosphate (NADPH). Lysates of EA.hy926 cells reduce alpha-lipoic acid using both NADPH and nicotinamide adenine dinucleotide (NADH) as electron donors, although NADPH-dependent reduction is about twice that due to NADH. NADPH-dependent alpha-lipoic acid reduction is mostly due to thioredoxin reductase. Pre-incubation of cells with alpha-lipoic acid increases their capacity to reduce extracellular ferricyanide, to recycle intracellular dehydroascorbic acid to ascorbate, to decrease reactive oxygen species generated by redox cycling of menadione, and to generate nitric oxide. These results show that alpha-lipoic acid enhances both the antioxidant defenses and the function of endothelial cells.

Glucose uptake and metabolism by cultured human skeletal muscle cells: rate-limiting steps

To use primary cultures of human skeletal muscle cells to establish defects in glucose metabolism that underlie clinical insulin resistance, it is necessary to define the rate-determining steps in glucose metabolism and to improve the insulin response attained in previous studies. We modified experimental conditions to achieve an insulin effect on 3-O-methylglucose transport that was more than twofold over basal. Glucose phosphorylation by hexokinase limits glucose metabolism in these cells, because the apparent Michaelis-Menten constant of coupled glucose transport and phosphorylation is intermediate between that of transport and that of the hexokinase and because rates of 2-deoxyglucose uptake and phosphorylation are less than those of glucose. The latter reflects a preference of hexokinase for glucose over 2-deoxyglucose. Cellular NAD(P)H autofluorescence, measured using two-photon excitation microscopy, is both sensitive to insulin and indicative of additional distal control steps in glucose metabolism. Whereas the predominant effect of insulin in human skeletal muscle cells is to enhance glucose transport, phosphorylation, and steps beyond, it also determines the overall rate of glucose metabolism.

Functional interaction between the N- and C-terminal halves of human hexokinase II

Mammalian hexokinases (HKs) I-III are composed of two highly homologous approximately 50-kDa halves. Studies of HKI indicate that the C-terminal half of the molecule is active and is sensitive to inhibition by glucose 6-phosphate (G6P), whereas the N-terminal half binds G6P but is devoid of catalytic activity. In contrast, both the N- and C-terminal halves of HKII (N-HKII and C-HKII, respectively) are catalytically active, and when expressed as discrete proteins both are inhibited by G6P. However, C-HKII has a significantly higher Ki for G6P (KiG6P) than N-HKII. We here address the question of whether the high KiG6P of the C-terminal half (C-half) of HKII is decreased by interaction with the N-terminal half (N-half) in the context of the intact enzyme. A chimeric protein consisting of the N-half of HKI and the C-half of HKII was prepared. Because the N-half of HKI is unable to phosphorylate glucose, the catalytic activity of this chimeric enzyme depends entirely on the C-HKII component. The KiG6P of this chimeric enzyme is similar to that of HKI and is significantly lower than that of C-HKII. When a conserved amino acid (Asp209) required for glucose binding is mutated in the N-half of this chimeric protein, a significantly higher KiG6P (similar to that of C-HKII) is observed. However, mutation of a second conserved amino acid (Ser155), also involved in catalysis but not required for glucose binding, does not increase the KiG6P of the chimeric enzyme. This resembles the behavior of HKII, in which a D209A mutation results in an increase in the KiG6P of the enzyme, whereas a S155A mutation does not. These results suggest an interaction in which glucose binding by the N-half causes the activity of the C-half to be regulated by significantly lower concentrations of G6P.

GLUT1 is adequate for glucose uptake in GLUT2-deficient insulin-releasing beta-cells

GLUT2 may play an important role in pancreatic beta-cell glucose metabolism. A decrease in glucose uptake due to underexpression of GLUT2 has been considered as the cause of beta-cell dysfunction in diabetes with different pathogenesis. However, this view has been challenged by recent studies, in which the underexpression of GLUT2 was not accompanied by a decrease in glucose uptake. Our present aim is to evaluate the presumed importance of GLUT2 in maintaining the efficiency of beta-cell glucose uptake. We studied the kinetic characteristics of 3-O-methylglucose uptake in two beta-cell lines. One of these is the beta TC3 cell line which expresses GLUT1 and the other is the beta HC9 cell line which expresses both GLUT1 and GLUT2. Under equilibrium exchange conditions, 3-O-methylglucose transport in these two cell lines showed similar values of K(m) and V(max). The apparent IC50 of cytochalasin B for inhibiting 3-O-methylglucose transport in beta HC9 cells was nine times as high as in beta TC3 cells, indicating that GLUT1 is the critically important glucose transporter in the beta TC3 cell line and GLUT2 in the beta HC9 cell line. In both cell lines, the rates of glucose uptake were at least three times as fast as that of glucose phosphorylation. Our results suggest that GLUT1 is able to compensate for GLUT2 loss as it occurs in beta TC3 and maintains a commensurately high capacity of glucose uptake to sustain glucose metabolism in pancreatic beta-cells.

Functional organization of mammalian hexokinase II. Retention of catalytic and regulatory functions in both the NH2- and COOH-terminal halves

The mammalian hexokinase (HK) family includes three closely related 100-kDa isoforms (HKI-III) that are thought to have arisen from a common 50-kDa precursor by gene duplication and tandem ligation. Previous studies of HKI indicated that a glucose 6-phosphate (Glu-6-P)-regulated catalytic site resides in the COOH-terminal half of the molecule and that the NH2-terminal half contains only a Glu-6-P binding site. In contrast, we now show that proteins representing both halves of human and rat HKII have catalytic activity and that each is inhibited by Glu-6-P. The intact enzyme and the NH2- and COOH-terminal halves of the enzyme each increase glucose utilization when expressed in Xenopus oocytes. Mutations corresponding to either Asp-209 or Asp-657 in the intact enzyme completely inactivate the NH2- and COOH-terminal half enzymes, respectively. Mutation of either of these sites results in a 50% reduction of activity in the 100-kDa enzyme. Mutation of both sites results in a complete loss of activity. This suggests that each half of the HKII molecule retains catalytic activity within the 100-kDa protein. These observations indicate that HKI and HKII are functionally distinct and have evolved differently.

Ascorbate recycling in human erythrocytes: role of GSH in reducing dehydroascorbate

Human erythrocytes regenerate ascorbate from its oxidized product, dehydroascorbate. The extent to which such ascorbate recycling occurs by a GSH-dependent mechanism was investigated. In the presence of glucose, erythrocytes took up over 90% of extracellular [14C]dehydroascorbate and rapidly converted it to [14C]ascorbate, which was trapped within the cells. Dehydroascorbate uptake and reduction was not associated with generation of a monoascorbyl free radical intermediate. Uptake and reduction of dehydroascorbate by glucose-depleted erythrocytes coordinately decreased GSH and raised GSSG concentrations in erythrocytes. This effect was reversed by D-glucose, but not by L-lactate. Conversely, depletion of cellular GSH decreased the ability of cells to recycle dehydroascorbate to ascorbate, as reflected in the extent to which cells were able to reduce extracellular ferricyanide. Monoascorbyl free radical was formed during the reduction of extracellular ferricyanide, indicating that one electron transfer steps were involved in this process. In GSH-depleted cells, addition of L-lactate as an energy source for glycolysis-dependent NADH regeneration did cause a partial recovery of the ability of cells to reduce ferricyanide. However, in resealed erythrocyte ghosts containing either 4 mM GSH or 400 mu M NADH, only the GSH-containing ghosts supported regeneration of ascorbate from added dehydroascorbate. These results suggest that in human erythrocytes ascorbate regeneration from dehydroascorbate is largely GSH dependent, and that it occurs through either enzymatic or nonenzymatic reactions not involving the monoascorbyl free radical.

Regulation of hexokinase II gene transcription and glucose phosphorylation by catecholamines, cyclic AMP, and insulin

The hexokinases, by converting glucose to glucose-6-phosphate, help maintain the downhill gradient that results in movement of glucose into cells through the facilitative glucose transporters. GLUT4 and hexokinase (HK) II are the major transporter and hexokinase isoforms in skeletal muscle, heart, and adipose tissue, wherein insulin promotes glucose utilization. To understand whether hormones influence the contribution of phosphorylation to cellular glucose utilization, we investigated the effects that catecholamines, cyclic AMP (cAMP), and insulin have on HKII gene expression in cells representative of muscle (L6 cells) and brown (BFC-1B cells) and white (3T3-F442A cells) adipose tissues. Isoproterenol or the cAMP analog 8-chlorophenylthio-cAMP selectively increase HKII gene transcription in L6 cells, as does insulin (Printz RL, Koch S, Potter LP, O'Doherty RM, Tiesinga JJ, Moritz S, Granner DK: Hexokinase II mRNA and gene structure, regulation by insulin, and evolution. J Biol Chem 268:5209-5219, 1993), and cause a concentration- and time-dependent increase of HKII mRNA in both muscle and fat cell lines without changing HKI mRNA. Isoproterenol and insulin also increase the rate of synthesis of HKII protein and increase glucose phosphorylation and glucose utilization in L6 cells.

Ascorbic acid recycling enhances the antioxidant reserve of human erythrocytes

The role of ascorbate transport and metabolism in the response of human erythrocytes to an extracellular oxidant stress was investigated. Rates of entry and exit of [14C]dehydroascorbate from erythrocytes were more than 10-fold greater than those of [14C]ascorbate. Both the reduced and oxidized forms of the vitamin were transported largely by the glucose transporter. Inside erythrocytes, dehydroascorbate was converted to ascorbate, increasing intracellular ascorbate concentrations 2-3-fold over those in the medium. In such ascorbate-loaded cells, the membrane-impermeant oxidant ferricyanide induced a transmembrane oxidation of intracellular ascorbate to dehydroascorbate. The latter escaped the cells on the glucose transporter, which resulted in a halving of the net entry of [14C]dehydroascorbate in the presence of ferricyanide. Treatment of ascorbate-loaded cells with H2O2 and Cu2+ also oxidized ascorbate and induced efflux of [14C]dehydroascorbate. Ferricyanide-dependent intracellular oxidation of ascorbate resulted in a corresponding reduction of extracellular ferricyanide, which served as an integrated measure of ascorbate recycling. Ferricyanide reduction was proportional to the loading concentration of dehydroascorbate and was enhanced when loss of dehydroascorbate from cells was decreased, either by blockade of the glucose transporter or by concentrating the cells. Selective depletion of cellular ascorbate lowered rates of ferricyanide reduction by two-thirds, suggesting that ascorbate rather than NADH is the major donor for the transmembrane ferricyanide oxidoreductase activity. On the basis of the ascorbate-dependent rate of ferricyanide reduction, erythrocytes at a 45% hematocrit can regenerate the ascorbic acid present in whole blood every 3 min. Erythrocyte ascorbate recycling may thus contribute more to the antioxidant reserve of blood than is evident from plasma ascorbate concentrations alone.

Two regions of GLUT 2 glucose transporter protein are responsible for its distinctive affinity for glucose

The glucose transporter in the hepatocyte and pancreatic beta-cell (GLUT 2) has a lower affinity for glucose than other members of the glucose transporter family. To investigate the molecular mechanism for the distinctive affinity of GLUT 2 for glucose, we expressed chimeric GLUT 2 and GLUT 4 proteins in Xenopus oocytes and measured 3-O-methyl-D-glucose transport. In the oocyte system, GLUT 2 had a Km of 31.8 +/- 2.8 mM for 3-O-methyl-D-glucose, whereas GLUT 4 had a Km of 7.2 +/- 2.4 mM under equilibrium exchange conditions. GLUT 4/GLUT 2 chimera that contained the intracellular loop and transmembrane domains 7-12 of GLUT 2 (amino acids 239-497) had a Km similar to that of wild-type GLUT 2. A GLUT 4/GLUT 2 chimera in which the COOH-terminal 30 amino acids of GLUT 4 were replaced with the corresponding region of GLUT 2 had a 2-fold higher Km than GLUT 4, but still had a much lower Km than GLUT 2. These results indicate that both transmembrane domains 7-12 and the COOH-terminus of the protein are responsible for the distinctive glucose affinity of GLUT 2.

Ascorbate is the major electron donor for a transmembrane oxidoreductase of human erythrocytes

Ascorbic acid is an important antioxidant in human blood. Erythrocytes contribute to the antioxidant capacity of blood by regenerating ascorbate and possibly by exporting ascorbate-derived reducing equivalents through a transmembrane oxidoreductase. The role of ascorbate as an electron donor to the latter enzyme was tested in human erythrocytes and ghosts using nitroblue tetrazolium as an electron acceptor. Although nitroblue tetrazolium was not directly reduced by ascorbate, erythrocyte ghosts facilitated reduction of nitroblue tetrazolium in the presence of ascorbate and ascorbate derivatives containing a reducing double bond. The resulting blue monoformazan product was deposited directly in ghost membranes. Ascorbate-induced monoformazan deposition showed several features of an enzyme-mediated process, including hyperbolic dependence on substrate and acceptor concentrations, as well as sensitivity to enzyme proteolysis, detergent solubilization, and sulfhydryl reagents. Incubation of intact erythrocytes with nitroblue tetrazolium caused deposition of the monoformazan in ghost membranes prepared from the cells. This deposition reflected the intracellular ascorbate content and was inhibited by extracellular ferricyanide, a known electron acceptor for the transmembrane oxidoreductase. Although nitroblue tetrazolium did not cross the cell membrane, like the cell-impermeant ferricyanide, it oxidized intracellular [14C]ascorbate to [14C]dehydroascorbate, which then exited the cells. In resealed ghosts, both monoformazan deposition and ferricyanide reduction were proportional to the intravesicular ascorbate concentration. NADH was only about half as effective as a donor for the enzyme as ascorbate in both open and resealed ghosts. These results suggest that not only can ascorbate donate electrons to a transmembrane oxidoreductase, but that it may be the major donor in intact erythrocytes.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Coexpression of glucose transporters and glucokinase in Xenopus oocytes indicates that both glucose transport and phosphorylation determine glucose utilization

A Xenopus oocyte expression system was used to examine how glucose transporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affect glucose utilization. Uninjected oocytes and low rates of both glucose transport and phosphorylation; expression of GLUT 2 or GLUT 3 increased glucose phosphorylation approximately 20-fold by a low Km, endogenous hexokinase at glucose concentrations < or = 1 mM, but not at higher glucose concentrations. Coexpression of functional GK isoforms with GLUT 2 or 3 increased glucose utilization approximately an additional two- to threefold primarily at the physiologic glucose concentrations of 5-20 mM. The Km for glucose of both the hepatic and beta cell isoforms of GK, determined in situ, was approximately 5-10 mM when coexpressed with either GLUT 2 or GLUT 3. The increase in glucose utilization by coexpression of GLUT 3 and GK was dependent upon glucose phosphorylation since two missense GK mutations linked with maturity-onset diabetes, 182: Val-->Met and 228:Thr-->Met, did not increase glucose utilization despite accumulation of both a similar amount of immunoreactive GK protein and glucose inside the cell. Coexpression of a mutant GK and a normal GK isoform did not interfere with the function of the normal GK enzyme. Since the coexpression of GK and a glucose transporter in oocytes resembles conditions in the hepatocyte and pancreatic beta cell, these results indicate that increases in glucose utilization at glucose concentrations > 1 mM depend upon both a functional glucose transporter and GK.

Coupling of glucose transport and phosphorylation in Xenopus oocytes and cultured cells: determination of the rate-limiting step

The initial events in glucose metabolism by all cells are the transport and phosphorylation of glucose. To quantify the relative contributions of these two processes to overall glucose utilization, we have developed an experimental approach for their in situ measurement as parallel processes. The method is based on the use of intracellular [2-3H]glucose as a substrate for both the transporter and hexokinase, and involves simultaneous measurement of [2-3H]glucose efflux and of 3H2O released by phosphorylation. The Xenopus oocyte expression system was used to test the method, since in these cells transport and phosphorylation activities can be regulated by expression of mRNA or injection of foreign protein. Oocytes microinjected with [2-3H]glucose showed no release of injected glucose, but did have saturable phosphorylation kinetics, with a Km of 40 microM and a Vmax of 0.1 nmol/min/oocyte. Co-injection of yeast hexokinase increased glucose phosphorylation by five-fold. Expression of human glucose transporter (GLUT1) mRNA resulted in a 25-30-fold increase in the rate of saturable efflux of microinjected glucose compared to control oocytes. The kinetics of transport and phosphorylation of [2-3H]glucose were analyzed by a multiple curve-fitting program that provided estimates of kinetic coefficients for both processes from a single time course. The analysis showed that expression of GLUT1 shifted the rate-limiting step in glucose utilization from transport to phosphorylation. A similar shift occurred at a three-fold lower extracellular concentration of 2-deoxyglucose. In a pancreatic beta cell line both transport and phosphorylation showed high Km values, with phosphorylation as the limiting step. The in situ measurement of glucose transport and phosphorylation as parallel processes should be useful in defining the relative contributions of each step to overall glucose metabolism in other cell and tissue models.

Induction of aP2 gene expression by nonmetabolized long-chain fatty acids

Long-chain fatty acids (FA) have been shown to regulate expression of the gene for the adipocyte FA-binding protein aP2. We examined whether this effect was exerted by FA themselves or by a FA metabolite. The alpha-bromo derivative of palmitate, an inhibitor of FA oxidation, was synthesized in the radioactive form, and its metabolism was investigated and correlated with its ability to induce aP2 in Ob1771 preadipocytes. alpha-Bromopalmitate was not utilized by preadipocytes. It was not cleared from the medium over a 24-hr period and was not incorporated into cellular lipids. Short incubations indicated that alpha-bromopalmitate exchanged across the preadipocyte membrane but remained in the free form inside the cell. In line with this, preadipocyte homogenates did not activate alpha-bromopalmitate to the acyl form. However, although it was not metabolized, bromopalmitate was much more potent than native FA in inducing aP2 gene expression. Induction exhibited the characteristics previously described for native FA, indicating that a similar if not identical mechanism was involved. The data indicated that induction of aP2 was exerted by unprocessed FA. Finally, in contrast to preadipocytes, adipocytes metabolized bromopalmitate. This reflected increased activity with cell differentiation of a palmitoyl-CoA synthase that could activate palmitate and bromopalmitate at about one-fifth the rate for palmitate. In preadipocytes, the predominant fatty-acyl-CoA synthase, arachidonyl-CoA synthase, had very low affinity for both FA. Increased activity of the palmitoyl-CoA synthase, which has a wider substrate range, is likely to be important for initiation of lipid deposition.

Transport and metabolism of glucose in an insulin-secreting cell line, beta TC-1

Kinetic characteristics of glucose transport and glucose phosphorylation were studied in the islet cell line beta TC-1 to explore the roles of these processes in determining the dependence of glucose metabolism and insulin secretion on external glucose. The predominant glucose transporter present was the rat brain/erythrocyte type (Glut1), as determined by RNA and immunoblot analysis. The liver/islet glucose transporter (Glut2) RNA was not detected. The functional parameters of zero-trans glucose entry were Km = 9.5 +/- 2 mM and Vmax = 15.2 +/- 2 nmol min-1 (microL of cell water)-1. Phosphorylation kinetics of two hexokinase activities were characterized in situ. A low-Km (0.036 mM) hexokinase with a Vmax of 0.40 nmol min-1 (microL of cell water)-1 was present along with a high-Km (10 mM) hexokinase, which appeared to conform to a cooperative model with a Hill coefficient of about 1.4 and a Vmax of 0.3 nmol min-1 (microL of cell water)-1. Intracellular glucose at steady state was about 80% of the extracellular glucose from 3 to 15 mM, and transport did not limit metabolism in this range. In this static (nonperifusion) system, 2-3 times more immunoreactive insulin was secreted into the medium at 15 mM glucose than at 3 mM. The dependence of insulin secretion on external glucose roughly paralleled the dependence of glucose metabolism on external glucose. Simulations with a model demonstrated the degree to which changes in transport activity would affect intracellular glucose levels and the rate of the high-Km hexokinase (with the potential to affect insulin release).

Evidence that downregulation of hexose transport limits intracellular glucose in 3T3-L1 fibroblasts

Measurements of initial glucose entry rate and intracellular glucose concentration in cultured cells are difficult because of rapid transport relative to intracellular volume and a substantial extracellular space from which glucose cannot be completely removed by quick exchanges of medium. In 3T3-L1 cells, we obtained good estimates of initial entry of [14C]methylglucose and D-[14C]glucose with 1) L-[3H]glucose as an extracellular marker together with the [14C]glucose or [14C]methylglucose in the substrate mixture, 2) sampling times as short as 2 s, 3) ice-cold phloretin-containing medium to stop uptake and rinse away the extracellular label, and 4) nonlinear regression of time courses. Methylglucose equilibrated in two phases--the first with a half-time of 1.7 s and the second with a half-time of 23 s; it eventually equilibrated in an intracellular space of 8 microliters/mg protein. Entry of glucose remained almost linear for 10 s, making its transport kinetics easier to study (Km = 5.7 mM, Vmax = 590 nmol.s-1.ml-1 cell water). Steady-state intracellular glucose concentration was 75-90% of extracellular glucose concentration. Cells grown in a high-glucose medium (24 mM) exhibited a 67% reduction of glucose-transport activity and a 50% reduction of steady-state ratio of intracellular glucose to extracellular glucose.

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)

Activation energy of the slowest step in the glucose carrier cycle: break at 23 degrees C and correlation with membrane lipid fluidity

Glucose transport in the rat erythrocyte is subject to feedback regulation by sugar metabolism at high but not at low temperatures [Abumrad et al. (1988) Biochim. Biophys. Acta 938, 222-230]. This indicates that temperature, which is known to alter membrane fluidity, also alters sensitivity of transport to regulation. In the present work, we have investigated a possible correlation between the effects of temperature on rate-limiting steps of glucose transport and on membrane fluidity. The dependences of methylglucose efflux and influx on cis and trans methylglucose concentrations were studied at temperatures between 17 and 37 degrees C. Membrane fluidity was monitored over the same temperature range by using electron paramagnetic resonance spectroscopy. External sugar did not affect efflux, and the Km and Vmax of sugar exit were respectively the same as the Km and Vmax of equilibrium exchange. These Km's were relatively temperature independent, but the Vmax's increased sharply with temperature. The Km and Vmax of methylglucose entry were respectively much lower than the Km and Vmax of exit and exchange. Consistent with the above, intracellular sugar greatly enhanced sugar influx, and did so by increasing the influx Vmax without affecting the influx Km. Both lines of evidence indicated that the conformational change of the empty sugar-binding site from in-facing to out-facing orientation is the rate-limiting step of sugar entry into the rat erythrocyte. This was the case at all temperatures; however, the discrepancies of coefficients declined significantly with increasing temperature.2+ The temperature dependence of the slowest step (change from in- to out-facing empty carrier) was evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin antagonism of catecholamine stimulation of fatty acid transport in the adipocyte. Studies on its mechanism of action

Insulin at physiological concentrations can suppress catecholamine activation of the membrane transport of long chain fatty acids in the adipocyte. We have previously shown that the stimulatory effect of catecholamines was mediated by a beta-receptor interaction and cAMP (Abumrad, N.A., Park, C.R., and Whitesell, R. R. (1986) J. Biol. Chem. 261, 13082-13086). In this study we have investigated the mechanism of insulin action to antagonize transport activation. Fatty acid transport was stimulated using different cAMP derivatives with varying susceptibilities to hydrolysis by the cAMP-degrading enzyme phosphodiesterase. Insulin was effective in antagonizing the effect of cAMP analogs which were good substrates for the phosphodiesterase and failed to suppress the effect of those which were poorly hydrolyzed by the enzyme. Addition of increasing concentrations (1-100 microM) of the phosphodiesterase inhibitor methylisobutylxanthine (MIX) to norepinephrine (0.1 microgram/ml) gradually abolished insulin's antagonism. Insulin was completely ineffective in inhibiting stimulation by norepinephrine and 20 microM methylisobutylxanthine. Also consistent with involvement of cAMP lowering in insulin action was the finding that adenosine removal greatly diminished insulin's responsiveness. Treatment of cells with adenosine deaminase (1 unit/ml) enhanced the effect of norepinephrine by about 30%. A 10-fold higher range of insulin concentrations was then required to produce inhibition of fatty acid transport. The effect of adenosine removal was reversed by addition of phenylisopropyladenosine (500 nM), which is resistant to hydrolysis by the deaminase. Finally, exposure of insulin-treated cells (1 nM for 5 min) to dinitrophenol (1 mM for 5 min) reversed insulin action, consistent with reports of reversal of insulin's activation of the phosphodiesterase. In conclusion, our studies support the involvement of cAMP lowering in insulin's antagonism of fatty acid transport stimulation in the adipocyte.

Temperature dependence of glucose transport in erythrocytes from normal and alloxan-diabetic rats

Alloxan diabetes increased 3-O-methylglucose transport rates in rat red blood cells (RBC) at temperatures below 30 degrees C and decreased them above 30 degrees C. Preincubation of RBC from control rats with 20 mM glucose, 3-O-methylglucose, 2-deoxyglucose or xylose greatly elevated transport at 14 degrees C by increasing Vmax. The effect was slight at 40 degrees C. Preincubation with glucose or deoxyglucose alone caused a 50% depression of transport rates at 40 degrees C as a result of a rise in the Km, which is similar to findings in cells from alloxan-diabetic rats. Measurement of intracellular glucose metabolites suggested inhibition of glycolysis in cells from diabetic rats and a positive correlation between the level of intracellular hexose monophosphates and transport inhibition. Membrane fatty-acid and cholesterol composition and membrane lipid-ordering as monitored by electron paramagnetic resonance were not altered by alloxan diabetes. It is concluded that intracellular sugar and sugar metabolism alter the temperature dependence of glucose transport kinetics. Glucose metabolism can feed back to inhibit transport by increasing the transport Km at physiological temperatures only.

Modulation of basal glucose transporter Km in the adipocyte by insulin and other factors

We have previously described experimental conditions where basal methylglucose transport in adipocytes exhibited an apparent Km of approximately 35 mM. Under those conditions insulin stimulated transport predominantly by decreasing the transport Km (Whitesell, R. R., and Abumrad, N. A. (1985) J. Biol. Chem. 260, 2894-2899). Our findings were in contrast with earlier reports that the Km of basal glucose transport was low (3-5 mM) and similar to that of transport in insulin-treated cells. In this study we have investigated the effect of different experimental conditions on the kinetics of basal glucose transport in adipocytes. When transport was assayed at 37 degrees C, cell agitation for 10 min prior to the transport assay decreased the basal Km from 35 to 12 mM. Deprivation of metabolic substrate produced a further reduction down to 2 mM. Refeeding starved cells with 1 mM glucose returned the Km back up to 12 mM in agitated cells and to 40 mM in stabilized cells. The effects of agitation to lower and of glucose to raise the basal Km were prevented by preincubating cells with dinitrophenol. Cell agitation or substrate lack did not alter the Vmax of basal transport and were without effect on both Km and Vmax in insulin-treated cells. The temperature dependencies of the kinetics of basal and stimulated transport were studied. A decrease in the assay temperature from 37 to 23 degrees C caused both basal Km and Vmax to drop proportionately from 25 to 5 mM, and 13 to 3.6 nmol/(microliter X min), respectively. In insulin-stimulated cells, only the Vmax was decreased (Km went from 3.5 to 3 mM, Vmax from 45 to 17 nmol/(microliter X min]. The results support the concept that experimental conditions can produce large changes in the Km of basal glucose transporters. Furthermore they explain why, under certain assay conditions (with temperatures around 23 degrees C or with deprivation of metabolic substrate), the effect of insulin on transport Km is not observed. Our data also suggest that basal transport characteristics do not persist in insulin-treated cells. We would propose that one of the actions of insulin (in addition to raising Vmax) is to change the characteristics of basal transporters by overriding metabolic factors which keep the Km high. Alternatively, insulin could cause the disappearance of basal transporters as new and different ones are recruited from intracellular stores.

Catecholamine activation of the membrane transport of long chain fatty acids in adipocytes is mediated by cyclic AMP and protein kinase

Membrane transport of long chain fatty acids in the isolated adipocyte can be stimulated 5-10-fold by epinephrine (Abumrad, N. A., Perry, P. R., and Whitesell, R. R. (1985) J. Biol. Chem. 260, 9969-9971). This study shows that isoproterenol and norepinephrine are more potent than epinephrine in activating the transport process. The stimulatory effect on transport is mediated by beta-receptor interaction and cAMP. This was shown by the following. alpha-Receptor agonists and antagonists were ineffective; methylisobutylxanthine at low concentration (3 microM) potentiated the effect of a suboptimal dose (0.01 microgram/ml) of epinephrine and was stimulatory at high concentration (100 microM) in the absence of epinephrine; and cAMP analogs were very effective activators. Involvement of the cAMP-dependent protein kinase was indicated by two lines of evidence. 1) Combinations of cAMP analogs which are specific for sites 1 and 2 of the protein kinase, respectively, had synergistic effects on fatty acid transport. Combinations of analogs specific for the same site were only additive in their effects. This is similar to the pattern of protein kinase activation in vitro and to that of lipolysis activation in the intact adipocyte (Beebe, S. J., Holloway, R., Rannels, S. R., and Corbin, J. D. (1984) J. Biol. Chem. 259, 3539-3547). 2) Treatment of cells with various metabolic poisons abolished the stimulatory effect of norepinephrine. The response of fatty acid transport to catecholamines showed multiple parallels with that documented for lipolysis except that it was much more rapid. This suggested that the transport process was a regulatory step in fatty acid mobilization. This interpretation is supported by the observation that basal Vmax for transport is much too slow to accommodate the rate of fatty acid release which is observed following stimulation of intact cells with adrenergic hormones.

Insulin antagonizes epinephrine activation of the membrane transport of fatty acids. Potential site for hormonal suppression of lipid mobilization

Membrane transport of long chain fatty acids in the isolated rat adipocyte can be strongly stimulated by epinephrine (Abumrad, N. A., Perry, P. R., and Whitesell, R. R. (1985) J. Biol. Chem. 260, 9969-9971). We now report that insulin at physiological concentrations can completely block or reverse the epinephrine effect. Insulin was optimally effective at a concentration of about 0.1 nM in inhibiting transport activation by 0.3 and 3 microM epinephrine (0.1 and 1.0 microgram/ml). High concentrations of insulin (above 1 nM) were generally less effective and this was particularly true at the highest dose of epinephrine (1.0 microgram/ml). The insulin effect was shown to be on the transport process since insulin inhibited epinephrine activation of transport in both directions (influx and efflux). No effect of insulin on basal transport was observed over a wide range of concentrations (0.01-10 nM). Insulin's antagonism of transport activation by epinephrine appeared dependent on ATP metabolism since it was abolished by preincubating the cells with dinitrophenol (1 mM). Dinitrophenol, however, could not reverse the insulin effect when exposure to the hormone preceded that to dinitrophenol, consistent with an action of insulin at the transport step. The data indicate that regulation of the membrane transport of fatty acids is a potential site for insulin's action to suppress lipid mobilization.

Stimulation by epinephrine of the membrane transport of long chain fatty acid in the adipocyte

In isolated rat adipocytes, epinephrine rapidly stimulates the transport of long chain fatty acid across the plasma membrane. At a concentration of unbound oleate of 0.1 microM ([fatty acid]/[albumin] = 1) and 5 min exposure to the hormone, the minimal effective concentration of epinephrine is 0.03 and the optimal concentration 0.3 microM (0.01 and 0.1 microgram/ml). The stimulated rates are 5-10-fold the basal rate of influx or efflux. The hormone effect is on the transport process specifically as shown by isolation of the product of transport in either direction as unesterified fatty acid and inhibition by the transport inhibitors phloretin and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. This effect of epinephrine on transport coordinates physiologically with lipase activation to bring about fatty acid release from adipose tissue.

Increased affinity predominates in insulin stimulation of glucose transport in the adipocyte

The kinetics for transport of glucose and 3-O-methylglucose (MeGlc) were determined in preparations of rat adipocytes characterized by low basal rates which could be increased consistently more than 30-fold by addition of insulin (10 nM). In basal cells, the Km of [14C]glucose uptake was about 75 mM. The Ki for glucose inhibition of [14C]MeGlc uptake was similarly high (105 mM). These results were further confirmed by studies of basal glucose consumption which remained linear up to the highest glucose concentration used (30 mM) and by measurements of MeGlc transport kinetics (net and equilibrium exchange Km were both about 35 mM). Basal glucose uptake was determined to be stereospecific and cytochalasin sensitive (greater than 90%) and thus could not be dismissed as nonmediated diffusion. Insulin treatment decreased the transport Km for glucose and MeGlc to one-tenth the values measured in basal cells. The Vmax for glucose was doubled and that for MeGlc quadrupled. We conclude that insulin brings about a great reduction in the Km of hexose transport in adipocytes. This can only be perceived if activation of basal cells by experimental manipulations is avoided. The decrease in Km is the major kinetic factor involved in the stimulation of glucose transport by insulin.

Glucose transport characteristics of quiescent thymocytes

Rat thymocyte populations contain "active" cells which equilibrate quickly with 3-O-methylglucose and "quiescent" cells which equilibrate slowly. Glucose transport stimuli make quiescent cells behave like active cells as regards glucose transport. Glucose transport in quiescent cells was inhibited by phloretin and cytochalasin B. Active cells were heterogeneous in their susceptibility to inhibition by 0.1 micron cytochalasin B, half of them being inhibited about 95%, while half were inhibited modestly. The methylglucose entry Km of quiescent cells was 3 times that of active and stimulated cells. The methylglucose entry Vmax of quiescent cells was one-eighth that of stimulated cells. A fixed internal methylglucose concentration increased the influx Km of quiescent cells but not that of active or stimulated cells. These findings and others are compatible with a model in which the carrier of quiescent cells is masked by an alteration (such as ligand binding) which is relatively specific for empty as opposed to sugar-loaded carrier and rather indifferent as to in versus out carrier orientation. The masking alteration stabilizes inward-facing and outward-facing unloaded carrier against both translocation and loading. The data fail to show evidence of a masking alteration of loaded carrier.

Mitogen-stimulated glucose transport in thymocytes. Possible role of Ca++ and antagonism by adenosine 3':5'-monophosphate

The plant lectin, concanavalin A (Con-A), and the ionophore, A-23187 (specific for divalent cations), stimulated glucose transport in rat thymocytes. Con-A stimulation developed more slowly and was somewhat less extensive than that of stimulation developed more slowly and was somewhat less extensive than that of A-23187. Both responses showed saturation dose dependencies. The two responses were poorly additive, suggesting that A-23187 may saturate regulatory processes shared by the two stimulatory mechanisms. Doses of methylisobutylxanthine (MIX) and prostaglandin E2 which raised adenosine 3':5'-monophosphate (cAMP) levels in these cells also antagonized the Con-A stimulation of glucose transport but did not inhibit basal glucose transport or the A-23187 stimulation. Dibutyryl-cAMP and 8-bromo-cAMP also natagonized Con-A stimulation without inhibiting basal glucose transport. MIX antagonized high Con-A doses about as strongly as it did low Con-A doses, suggesting that MIX did not compete in the Con-A binding step or other process saturable by Con-A. [3H-A1Con-A binding was not affected by MIX. The stimulatory effects of Con-A and A-23187 were reduced by reduction of Ca++ in the medium. Both Con-A and A-23187 enhanced 45Ca++ influx and cellular Ca++ content. The A-23187 dose, which was saturating for glucose transport stimulation, enhanced Ca++ influx and cellular Ca++ content more than did the Con-A dose which was saturating for glucose transport stimulation. The dose fo MIX which specifically antagonized Con-A stimulation of glucose transport proved also to reduce Ca++ influx and cellular Ca++ in the presence of Con-A but not in the presence of A-23187. Thus, glucose transport correlates rather well with cellular Ca++. These results are compatible with the view that Ca++ in a cellular compartment can promote glucose transport, the Con-A's enhancement of Ca++ entry contributes to its stimulation of glucose transport, and the MIX antagonized Con-A action at least partly by reducing Ca++ entry. The action of MIX is apparently mediated by cAMP.