Transport and phosphorylation of hexoses in normal and Rous sarcoma virus-transformed chick embryo fibroblasts

Increased levels of cyclins D1 and D3 after inhibition of gap junctional communication in astrocytes

We showed previously that the inhibition of gap junctional communication in astrocytes increased bromodeoxyuridine (BrdU) incorporation and promoted changes in the metabolic phenotype destined to fulfil the requirements of cell proliferation. In the present study we investigated the changes in the cell cycle of astrocytes promoted by the inhibition of intercellular communication through gap junctions. Thus, the presence of endothelin-1 and carbenoxolone, two gap junction uncouplers, promoted an increase in the percentage of astrocytes found in the S, G2 and M phases of the cell cycle, with a concomitant decrease in G0 and G1 phases. In addition, the levels of Ki-67, a protein present during all active phases of the cell cycle but absent from resting cells, increased after the inhibition of gap junctional communication. These effects were not observed when the inhibition of gap junctions was prevented with tolbutamide, indicating that the inhibition of gap junctional communication promotes the entry of astrocytes into the cell cycle. The passage of the cells from a quiescent state to the cell cycle is ultimately regulated by the degree of retinoblastoma phosphorylation. Inhibition of gap junctions increased the phosphorylation of retinoblastoma at Ser 780 but not at Ser 795 or Ser 807/811. In addition, the levels of cyclins D1 and D3 increased, whereas those of p21 and p27 were not significantly modified. Because D-type cyclins are key regulators of retinoblastoma protein phosphorylation, it is suggested that the phosphorylation of retinoblastoma protein at Ser 780, observed under our experimental conditions, is a consequence of the increase in the levels of cyclins D1 and D3. Our work provides evidence for the involvement of cyclins D1 and D3 as sensors of the inhibition of gap junctional communication in astrocytes.

The increase in gap junctional communication decreases the rate of glucose uptake in C6 glioma cells by releasing hexokinase from mitochondria

We have previously shown that the enhancement of glucose uptake caused by the inhibition of gap junctional communication is a consequence of the increase in astrocyte proliferation. Since C6 glioma cells are highly proliferative and are poorly coupled through gap junctions, we used these cells to investigate the effect of increasing gap junctional communication on the rate of glucose uptake. Previous work by us had shown that tolbutamide increases gap junctional communication in C6 glioma cells, as does dbcAMP, a classical activator of gap junctional communication. In this work, our results show that both tolbutamide and dbcAMP reduce the rate of glucose uptake in C6 glioma cells and that their effects are additive. The main glucose transporters expressed in C6 glioma cells are GLUT-1 and GLUT-3. Neither the expression nor the cellular localization of either GLUT-1 or GLUT-3 were modified by increasing gap junctional communication. The estimation of glucose uptake with 2-deoxyglucose includes not only glucose transport but also glucose phosphorylation, which in C6 glioma cells is mainly catalyzed by type I and type II hexokinase. Our results reveal that the increase in gap junctional communication caused by tolbutamide and dbcAMP is associated with a decrease in the activity of hexokinase. In agreement with this, tolbutamide and dbcAMP caused a rapid change in the localization of both type I and type II hexokinase, which were detached from the mitochondria to the cytosol.

Endothelin-1 stimulates the translocation and upregulation of both glucose transporter and hexokinase in astrocytes: relationship with gap junctional communication

We have previously shown that endothelin-1 increases glucose uptake in astrocytes. In the present work we investigate the mechanism through which endothelin-1 (ET-1) increases glucose uptake. Our results show that ET-1 activates a short-term and a long-term mechanism. Thus, ET-1 induced a rapid change in the localization of both GLUT-1 and type I hexokinase. These changes are probably aimed at rapidly increasing the entry and phosphorylation of glucose. In addition, ET-1 upregulated GLUT-1 and type I hexokinase and induced the expression of isoforms not normally expressed in astrocytes, such as GLUT-3 and type II hexokinase. These changes provide astrocytes with the machinery required to sustain a high rate of glucose uptake for a longer period of time. Our previous work had suggested that the effect of ET-1 on glucose uptake was associated with the inhibition of gap junctions. In this work, we compare the effect of ET-1 with that of carbenoxolone, a classical inhibitor of gap junction communication. Carbenoxolone increased glucose uptake to the same extent as ET-1 following the same mechanisms. Thus, carbenoxolone induced a rapid change in the localization of both GLUT-1 and type I hexokinase, upregulated GLUT-1 and type I hexokinase and induced the expression of GLUT-3 and type II hexokinase. When the inhibition of gap junction was prevented by tolbutamide, neither ET-1 nor carbenoxolone were able to increase the levels of GLUT-1, GLUT-3, type I hexokinase or type II hexokinase, indicating that these events are closely related to gap junctions.

Characterization of porin isoforms expressed in tumor cells

Mitochondria from malignant tumor cell lines show a higher capability for hexokinase binding than those from normal liver. To explore possible differences in hexokinase binding sites of mitochondria between tumor cells and normal liver, we characterized porin isoforms expressed in tumor cells. Cloning experiments on the three porin isoforms, VDAC1, VDAC2 and VDAC3 from malignant tumor cell line AH130 clearly showed that their primary structures were completely identical to those of the corresponding VDACs of normal liver cells. Possible expression of the fourth porin isoform in AH130 cells was excluded by degenerate primer-based RT-PCR. However, the transcript levels of the three VDAC isoforms in AH130 cells were significantly higher than those in normal liver. These results suggest that the high hexokinase-binding capability of malignant tumor cell mitochondria was not due to any structural difference, but due to a quantitative difference in binding sites.

Reassessment of FDG uptake in tumor cells: high FDG uptake as a reflection of oxygen-independent glycolysis dominant energy production

To determine appropriate use of 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) in the diagnosis of malignant tumors, the mechanism of enhanced FDG uptake in tumor cells was reassessed using in vitro cultured cell lines and 3H-deoxyglucose (DG), in combination with possible parameters of aerobic and anaerobic energy production. The high DG uptake in the tumor cells reflected the dependency of energy production on anaerobic glycolysis, and paradoxically on low levels of aerobic oxidative phosphorylation in mitochondria. We discuss here factors underlying anaerobic glycolysis in tumor cells.

Source of ATP for hexokinase-catalyzed glucose phosphorylation in tumor cells: dependence on the rate of oxidative phosphorylation relative to that of extramitochondrial ATP generation

We isolated highly intact and tightly coupled mitochondria from the rat ascites hepatoma cell line AH130 by disruption of the cell membrane by nitrogen cavitation. These isolated mitochondria were found to have essentially the same functional properties as rat liver mitochondria, but unlike the latter, hexokinase (HK) was bound to their membrane. Using the tumor mitochondrial preparation, we examined the source of ATP for phosphorylation of glucose by HK under conditions in which intra- and extramitochondrial ATP-generation systems operated separately or together. Results showed that the membrane-bound HK utilized ATP derived from the most efficiently operating ATP generation system, i.e., oxidative phosphorylation. However, when the rate of extramitochondrial ATP generation was much greater than that of oxidative phosphorylation, HK used ATP from the extramitochondrial ATP-generation system.

Glucose catabolism in cancer cells. Isolation, sequence, and activity of the promoter for type II hexokinase

One of the most characteristic phenotypes of rapidly growing cancer cells is their propensity to catabolize glucose at high rates. Type II hexokinase, which is expressed at high levels in such cells and bound to the outer mitochondrial membrane, has been implicated as a major player in this aberrant metabolism. Here we report the isolation and sequence of a 4.3-kilobase pair proximal promoter region of the Type II hexokinase gene from a rapidly growing, highly glycolytic hepatoma cell line (AS-30D). Analysis of the sequence enabled the identification of putative promoter elements, including a TATA box, a CAAT element, several Sp-1 sites, and response elements for glucose, insulin, cAMP, Ap-1, and a number of other factors. Transfection experiments with AS-30D cells showed that promoter activity was enhanced 3.4-, 3.3-, 2.4-, 2.1-, and 1.3-fold, respectively, by glucose, phorbol 12-myristate 13-acetate (a phorbol ester), insulin, cAMP, and glucagon. In transfected hepatocytes, these same agents produced little or no effect. The results emphasize normal versus tumor cell differences in the regulation of Type II hexokinase and indicate that transcription of the Type II tumor gene may occur independent of metabolic state, thus, providing the cancer cell with a selective advantage over its cell of origin.

Nucleotide sequence of the 5'-flanking region of the rat type II hexokinase gene

In a previous study, we found that the steady state transcript level of type II hexokinase was specifically and remarkably elevated in rat hepatoma AH130 cells. To determine the molecular mechanism of transcriptional control of the type II hexokinase gene, we examined the nucleotide sequence of its 5'-flanking region and analyzed putative transcription factor binding sites. We also examined the type II hexokinase promoter activity by the chloramphenicol acetyltransferase (CAT) assay.

Deoxyglucose lumped constant estimated in a transplanted rat astrocytic glioma by the hexose utilization index

The lumped constant (LC) for calculating the regional glucose (glc) metabolic rate by the deoxyglucose (DG) method was estimated in a transplanted rat glioma and normal rat brain. First, the hexose utilization index (HUI) was measured at 1.5, 3.0, and 4.5 min in right hemisphere glioma implants and uninvolved contralateral hemisphere following bolus intravenous injections of [3H]DG and [14C]glucose. At these times, the glioma HUI values were 0.639, 0.732, and 0.712, respectively, and the coordinate left hemisphere values were 0.432, 0.449, and 0.418. Second, the volumes of distribution of DG and glucose were determined to be 0.436 and 0.235 in glioma implants and 0.402 and 0.237 in left hemisphere, respectively. Third, following simultaneous intracarotid injections of [3H]DG and [14C]glucose, the ratio K1/K1 was 1.1 in glioma grafts and 1.3 in left hemisphere. With these values for HUI, volume of distribution, and K1 ratio, the LC in this rat glioma was estimated to be 2.1 times higher than the left hemisphere LC (p less than 0.02). These results suggest that measurement of brain tumor CMRglc using a normal brain LC may significantly overestimate the true tumor CMRglc.

Hexokinase isoenzymes of RIN-m5F insulinoma cells. Expression of glucokinase gene in insulin-producing cells

We have analysed the pattern of expression of the hexokinase isoenzyme group in RIN-m5F insulinoma cells. Three hexokinase forms were resolved by DEAE-cellulose chromatography. The most abundant isoenzyme co-eluted with hexokinase type II from rat adipose tissue and displayed a Km for glucose of 0.15 mM, similar to the adipose-tissue enzyme. Hexokinase type II was in large part associated with a particulate subcellular fraction in RIN-m5F cells. The two other hexokinases separated by ion-exchange chromatography were an enzyme similar to hexokinase type I from brain and glucokinase (or hexokinase type IV). The latter isoenzyme was identified as the liver-type glucokinase by the following properties: co-elution with hepatic glucokinase from DEAE-cellulose and DEAE-Sephadex; sigmoid saturation kinetics with glucose with half-maximal velocity at 5.6 mM and Hill coefficient (h) of 1.54; suppression of enzyme activity by antibodies raised against rat liver glucokinase; apparent Mr of 56,500 and pI of 5.6, as shown by immunoblotting after one- and two-dimensional gel electrophoresis; peptide map identical with that of hepatic glucokinase after proteolysis with chymotrypsin and papain. These data indicate that the gene coding for hepatic glucokinase is expressed in RIN-m5F cells, a finding consistent with indirect evidence for the presence of glucokinase in the beta-cell of the islet of Langerhans. On the other hand, the overall pattern of hexokinases is distinctly different in RIN-m5F cells and islets of Langerhans, since hexokinase type II appears to be lacking in islets. Alteration in hexokinase expression after tumoral transformation has been reported in other systems.

The role of the mitochondrial outer membrane in energy metabolism of tumor cells

The outer mitochondrial membrane contains a pore structure which is composed of a 30,000 Da protein, porin. The pore has an internal diameter of 2 nm and exhibits a molecular-sieving exclusion limit between 3000 and 6000 Da. These pores, therefore, provide the exit/entrance port for metabolites moving between mitochondria and the cytosol. Hexokinase binds to porin on the outer surface of mitochondria. The location of hexokinase has evoked a number of theories in which bound hexokinase is given a central role in regulating glycolysis, and, perhaps, the metabolic communication between oxidative and glycolytic metabolism. This is of particular importance in rapidly growing tumor cells in which the aerobic production of lactate and hexokinase activity are highly induced. In the present paper, we summarize the suggested roles of the outer membrane and bound hexokinase in regulation glycolysis of tumor cells. Experiments attempting to elucidate the role of hexokinase binding in the regulation of tumor cell metabolism are presented.

Analysis of hexose transport in untransformed and sarcoma virus-transformed mouse 3T3 cells by photoaffinity binding of cytochalasin B

The effect of simian virus 40 transformation on the hexose transport system in mouse embryo fibroblast Swiss 3T3 cells was examined. The concentration of hexose transporters was estimated by measuring D-glucose-inhibitable cytochalasin B binding. The binding of cytochalasin B to the plasma membranes of simian virus 40-transformed mouse 3T3 cells (SV3T3 cells) was significantly greater than that of 3T3 cells. On the other hand, cytochalasin B binding to the microsomal membranes of SV3T3 cells was decreased, and the total amount of binding to plasma and microsomal membranes was not significantly changed in both cell lines. The electrophoretic analysis demonstrated that both hexose-transporter components of Mr 46 000 and Mr 58 000 affinity labeled were responsible for an increase in the hexose transport by viral transformation. These results suggested that the higher hexose-transport activity of transformed cells is caused by a redistribution of transporter from intracellular membranes to plasma membranes.

'Abnormality of 2-deoxyglucose uptake kinetics in fibroblasts at low concentrations

2-deoxyglucose uptake rates at low sugar concentrations (less than 500 microM) appeared to be lower than those predicted by the Michaelis-Menten model which correctly described higher concentrations. This phenomenon which we will call concentration-dependent transport lag, was also observed for L-glucose uptake which suggest that this phenomenon is carrier-independent. A model involving the perimembrane space is developed which, for L-glucose, gives k1 = 0.931 +/- 0.072 X 10(-6) l X mg protein-1 X minute-1, k2 = 2.97 +/- 0.19 X 10(-7) l X mg protein-1 X minute-1 and So = 88,8 +/- 4,3 microM; where k1 is the diffusion constant in the cell membrane, k2 is the diffusion constant in the perimembrane space and So the sugar concentration required in the external medium in order to provide an équivalent sugar concentration in the transport carrier area.

Glycogen regulation in LPS-stimulated murine splenocytes

Enzymatic glycogen regulation in mouse splenocytes cultured in vitro with and without LPS, was studied from 0-72 h. Increased [3H]glucose uptake and hexokinase activity demonstrated the activation of cells treated with LPS. There was a greater time-dependent increase of cellular glycogen content in LPS-stimulated cells as compared to control. Glycogen synthetase I in LPS-stimulated cells increased about 200% above control cells to a plateau at 48 h, while in unstimulated cells there was little increase throughout. Glycogen synthetase D increased continually to 72 h in both groups. In the stimulated cells, phosphorylase increased only 90% above control cells up to 48 h. It was concluded that the increased glycogen content of LPS-stimulated cells seen at 48 h may result from an increase in both glycogen synthetase I and D activity compared to lesser increase in hydrolysis. However, between 48 and 72 h, the period of RNA and DNA increased synthesis, the glycogen content of stimulated cells did not increase further, consistent with the observation that synthetase I activity remained constant and synthetase D decreased. Thus, following mitogenic stimulation, the net effect of the enzymatic regulation is to increase cellular glycogen, as an energy source for subsequent events.

Glycerol transport and phosphorylation by rat hepatocytes

The entry of glycerol into isolated rat hepatocytes appears to be catalyzed by a specific carrier. At a physiological concentration of 0.1 mM, glycerol utilization is rate limited by the permeation step. Intracellular glycerol is trapped by an excess of glycerol kinase, which has a higher apparent affinity for the substrate than that of the membrane carrier. The entry of glycerol into the hepatocytes is highly sensitive to inhibition by monoacetin and cytochalasin B, but not by DL-1,2-propanediol, erythritol, D-glucose, D-galactose, D-mannose, or D-fructose.

Polyamines stimulate the binding of hexokinase type II to mitochondria

Spermine and spermidine enhanced the binding of hexokinase isoenzyme type II to mitochondria, both of which were prepared from Ehrlich-Lettre hyperdiploid ascites tumor cells, at much lower concentrations than Mg2+. Chymotrypsin-treated hexokinase II could not bind to the mitochondrial membrane in the presence of either spermine or Mg2+, indicating that the effect of spermine is not a nonspecific action, since the treatment of chymotrypsin cleaves only the region essential for the binding without any significant effect of the catalytic activity. Both spermine and Mg2+ antagonized the glucose 6-phosphate-induced release of mitochondria-bound hexokinase, and promoted the binding of the solubilized hexokinase II even in the presence of glucose 6-phosphate. However, inhibition of the activity of soluble hexokinase by glucose 6-phosphate was not reversed by spermine and Mg2+. Hexokinase II rebound to mitochondria with spermine and Mg2+ produced glucose 6-phosphate using ATP generated inside the mitochondria, and no difference was observed between the spermine- and Mg2+-rebound systems. Significance of the binding of hexokinase to mitochondria, especially with polyamines, is discussed with reference to high glycolytic rate in tumor cells.

Hexose sugar transport activity in a mouse fibroblast cell line temperature sensitive for expression of the transformed phenotype

A temperature-sensitive mouse fibroblast cell line was used to examine the relationship between hexose sugar uptake rates and the control of cell growth. The cell line (ts-H6-15) is a derivative of SV-3T3 cells, exhibiting a transformed phenotype at 32 degrees C and a normal phenotype at 39 degrees C. For cells actively growing at either temperature, a marked decrease in the rate of 3-O-methyl-D-glucose (3-O-MeG) transport is observed as cell population density increases. At cell population densities tested, 3-O-MeG transport rates (at a common assay temperature) were greater in H6-15 cells grown at 32 degrees C than at 39 degrees C, with the enhancement being maximal at the lowest cell densities. The effect of low serum-arrest on H6-15 cells revealed that cells growing at 39 degrees C arrest in G1, while cells at 32 degrees C stop more randomly throughout their cycle. Under conditions of low serum-arrest the rate of 3-O-MeG transport remained as high as in actively growing cells at both 32 degrees C and 39 degrees C. However, 2-deoxyglucose uptake rates were growth state-dependent at 39 degrees C, indicating perhaps metabolic as well as membrane-level control of sugar accumulation. These results further demonstrate that rates of hexose sugar transport by themselves are not always absolutely correlated with rates of cell proliferation and, thus, may be reliable predictors of cell growth potential.

Hexose transport in normal and SV40-transformed human endothelial cells in culture

The mechanism of glucose entry into human vascular endothelial cells was studied in monolayer cultures of normal (primary) and virally (SV40) transformed umbilical vein endothelium. Radioisotopic uptake studies with the glucose analogues 2-deoxy-D-glucose, and 3-O-methyl-D-glucose, and the nonmetabolizable stereoisomer L-glucose, indicated the presence of a saturable, stereospecific hexose carrier mechanism in both cell types. In other experiments with D-glucose and 3-O-methyl-D-glucose, the phenomenon of countertransport was demonstrable. Hexose transport was not affected by KCN, dinitrophenol, or ouabain, but was inhibited by phloretin and phlorizin in a pattern consistent with facilitated diffusion. Kinetic constants were obtained for both 2-deoxy-D-glucose and 3-O-methyl-D-glucose uptake. Similar Km values (range, 3.3-4.7 mM) were noted with normal and transformed cells, whereas the apparent Vmax was 0.56 nmol/microliter cytosol/minute for primary cells and 1.7-2.5 nmol/mu cytosol/minute for transformed cells. Under standard culture conditions, as well as following 18 hours of serum deprivation, insulin at concentrations up to 10(-5) M did not appear to influence hexose uptake in either cell type. Metabolism of 14C(U)-D-glucose to 14CO2 also was not stimulated by insulin. The presence of an insulin-insensitive, facilitated transport system for glucose in vascular endothelium has relevance for glucose metabolism in this tissue, and potentially for the association of certain vascular diseases (e.g., diabetic microangiopathy, atherosclerosis) with altered glucose homeostasis.

Hexose and amino acid transport by chicken embryo fibroblasts infected with temperature-sensitive mutant of Rous sarcoma virus. Comparison of transport properties of whole cells and membrane vesicles

The effect of transformation on hexose and amino acid transport has been studied using whole cells and membrane vesicles of chicken embryo fibroblasts infected with the temperature-sensitive mutant of the Rous sarcoma virus, TS-68. In whole cells, TS-68-infected chicken embryo fibroblasts cultured at the permissive temperature (37 degrees C) had a 2-fold higher rate of 2-deoxy-D-glucose uptake than the same cells cultured at the non-permissive temperature (41 degrees C). However, both the non-transformed and transformed cells had comparable rates of alpha-aminoisobutyric acid transport. Membrane vesicles, isolated from TS-68-infected chicken embryo fibroblasts cultured at 41 degrees C or 37 degrees C, displayed carrier-mediated, intravesicular uptake of D-glucose and alpha-aminoisobutyric acid. Membrane vesicles from TS-68-infected chicken embryo fibroblasts cultured at 37 degrees C had an approx. 50% greater initial rate of stereospecific hexose uptake than the membrane vesicles from fibroblasts cultured at 41 degrees C. The two types of membrane vesicle had similar uptake rates of alpha-aminoisobutyric acid. The results of hexose and amino acid uptake by the membrane vesicles correlated well with those observed with the whole cells. Km values for stereospecific D-glucose uptake by the membrane vesicles from TS-68-infected chicken embryo fibroblasts cultured at 41 and 37 degrees C were similar, but the V value was greater for the membrane vesicles from TS-68-infected cells cultured at 37 degrees C. Cytochalasin B competitively inhibited stereospecific hexose uptake in both types of membrane vesicle. These findings suggest that the membrane vesicles retained many of the features of hexose and amino acid transport observed in whole cells, and that the increased rate of hexose transport seen in the virally-transformed chicken embryo fibroblasts was due to an increase in the number or availability of hexose carriers.