Kinetic characteristics and regulation of hexose transport in a galactokinase-negative Chinese hamster fibroblast cell line: a good model for studies on sugar transport in cultured mammalian cells

Different Forms of Vanadate on Sugar Transport in Insulin Target and Nontarget Cells

The effects of several vanadates (ie, orthovanadate, pervanadate, and two stable peroxovanadium compounds) on basal and insulin-stimulated 2-DG transport in insulin target and nontarget cell lines are reported, herein. In nontarget cells, exposure to vanadates (5 x 10(-6) to 10(-4) mol/L) resulted in 2-DG transport stimulatory responses similar to those observed in 2-DG transport post exposure to 667 nmol/L insulin alone, or insulin in combination with vanadates. In 3T3-L1 adipocytes and L6 myotubes, exposure to a vanadate compound or 67 nmol/L insulin, stimulated 2-DG transport dramatically. Again, this effect on stimulated transport was similar to 2-DG transport post-treatment with the effective vanadates in combination with insulin. While pervanadate or stable peroxovanadates stimulated 2-DG transport at 10(-5) to 10(-6) mol/L, orthovanadate up to 10(-4) mol/L was not effective in stimulating 2-DG transport in any of the cell lines tested. The data indicate that the various peroxovanadates are clearly superior insulin mimetics while a more limited insulin mimesis is observed with orthovanadate over a wide variety of cell types.

Acylation-stimulating protein (ASP) regulates glucose transport in the rat L6 muscle cell line

Acylation-stimulating protein (ASP), a human plasma protein, is a potent stimulator of triglyceride synthesis and glucose transport in both human adipocytes and fibroblasts. The purpose of the present in vitro study was to examine the effect of ASP on glucose transport in muscle cells. ASP stimulated 2-deoxy-glucose transport (2-DG) in differentiated rat L6 myotubes in a time (30 min to 24 h) and concentration dependent manner (97% increase). The magnitude of the ASP effect on glucose transport was comparable to the time- and concentration-dependent effects seen with insulin (125% increase), but was additive to insulin, pointing to involvement of differential signalling pathways. ASP stimulation was dependent on cell differentiation in that glucose transport increased by only 12% in myoblasts, comparable to the effect of insulin in myoblasts (15% increase) demonstrating selective responsiveness of the differentiated myotubes to ASP and insulin. The mechanism for the ASP induced increase in glucose transport was also examined. ASP increased the Vmax for 2-DG transport by 183% (4.02 vs. 1.42 nmol/mg cell protein/30 s; ASP vs. Control, respectively). This could be explained by an increased translocation of glucose transporters (GLUT 1, GLUT 4 and GLUT 3) to the plasma membrane surface as demonstrated by Western analysis (+43% P < 0.05, +30% P < 0.05, and +49% P < 0.05, respectively). The effects of ASP were equal to those of insulin (+47%, +26% and +53% for GLUT 1, GLUT 4 and GLUT 3, respectively) and in all cases were paralleled by comparable glucose transport increases under the same incubation conditions. After long-term stimulation (24 h), Western analysis indicated that ASP had a permissive effect on insulin stimulated increases in total GLUT3 and GLUT4 cellular transporter content. These results suggest that muscle is also responsive to ASP and that ASP may play a role in glucose metabolism in both muscle and adipose tissue.

Evidence that modulation of glucose transporter intrinsic activity is the mechanism involved in the allose-mediated depression of hexose transport in mammalian cells

In serum starved V79 Chinese hamster lung fibroblast cells, replacement of D-glucose with D-allose resulted in a significant 38 +/- 18% (P < 0.05) reduction of 2-deoxy-D-glucose (2-DG) transport. Similarly, in a respiration-deficient mutant cell line (V79-G14), which has elevated 2-DG transport activity, D-allose reduced 2-DG transport by 59 +/- 18% (P < 0.05). [3H]D-allose uptake by V79 cells occurred slowly and was not inhibited by cytochalasin B, suggesting diffusion as the mode of D-allose entry. Western blot analysis using a rabbit polyclonal antibody to the human erythrocyte glucose transporter (GT) demonstrated that, in both cell lines, GT content and GT subcellular distribution were not significantly different in D-glucose vs. D-allose-treated cells. delta-Antibody, which has been shown to bind to exofacial epitopes of the GT (Harrison et al., 1990, J. Biol. Chem., 265:5793-5801), did not demonstrate any differences in surface binding to D-glucose vs. D-allose-treated intact V79 cells. D-allose treatment of 3T3 fibroblasts resulted in a similar decrease (72%) of 2-DG transport, however D-allose had no apparent effect on basal sugar transport in 3T3 adipocytes. These results suggest that D-allose reduces sugar transport through a modulation of the intrinsic activity of the GT, and that D-allose may act in a tissue-specific manner.

Mutant cell line demonstrating a block in insulin and insulin-like growth factor type 1 (IGF-1) induced mitogenesis

Recently, we have isolated a Chinese hamster cell variant (IV-A1-j) resistant to an insulin-diphtheria-A chain toxic conjugate (Leckett and Germinario: Cytotechnology [in press]. This cell line exhibited a decreased level of insulin binding, but normal growth in serum-containing medium when compared to the parental cell line (V-79). In this paper we further demonstrate that while IV-A1-j cells are capable of growing in serum-containing medium, they are insensitive to the mitogenic actions of either insulin or IGF-1. In contrast, epidermal growth factor (EGF) and/or alpha-thrombin (THR) generate a mitogenic effect in IV-A1-j cells comparable to that observed in the parental V-79 cells. The combination of EGF and/or THR with either insulin or IGF-1 results in an increase in V-79 cell growth above EGF and/or THR alone. On the other hand, insulin or IGF-1 in the presence of other mitogens did not stimulate further growth in IV-A1-j cells. While insulin binding was lower in IV-A1-j cells, internalization of 125I-insulin was not different in the two cell types. Additionally, insulin-stimulated glycogen synthesis and protein synthesis were not different in the two cell types. These observations are consistent with insulin and IGF-1 sharing a mitogenic signalling pathway in Chinese hamster fibroblasts and that this pathway is distinct from other growth factor signalling pathways. The fact that this pathway is defective in the IV-A1-j cell line indicates the potential usefulness of these cells in identifying a key step(s) in the insulin (IGF-1) mitogenic pathway.

Impaired hexose uptake by diploid skin fibroblasts from galactosaemic patients. Connection with cell growth and amino acid metabolism, and possible bearing on late-onset clinical symptoms

In skin fibroblasts of patients presenting with galactosaemia, either from galactose 1-phosphate uridyltransferase or galactokinase deficiency, a deficit in extracellular glucose utilization was observed. This deficit was constant over 3 weeks of continuous cell growth in a medium containing 5.5 mmol/L glucose as the only hexose, and homologous serum. Levels of glucose utilization by deficient skin fibroblasts were stable at about 65-70% of the glucose utilization of control normal skin fibroblasts. Cell morphology was normal, and cell growth was subnormal during this period. However, the energy provision appeared sufficient for cellular needs since cell growth in this glucose medium was observed not to depend on the presence of extracellular glutamine. In contrast, glutamine was required for growth of galactosaemic fibroblasts cultured in medium containing 5.5 mmol/L galactose. If expressed in many cell types, this impaired glucose uptake would be expected seriously to damage highly glucose-dependent tissues such as the central nervous system. This might be of relevance to the persistent neurological damage observed in many galactosaemic patients in spite of their compliance with an early strict galactose-free diet.

Construction of a toxic insulin molecule: selection and partial characterization of cells resistant to its killing effects

We have constructed an insulin-diphtheria hormono-toxin which migrates as a single 29 kd band on 10% SDS polyacrylamide gel electrophoresis. This corresponds to a one to one molar ratio of the diphtheria A-chain (23 kDa) and insulin (6 kDa) molecules. The diphtheria A-chain: insulin (DTaI) hormono-toxin demonstrates cytotoxicity in V-79 Chinese hamster cells exhibiting an LD50 of 1.1 x 10(-8) M, which is 22 x more potent than whole diphtheria toxin. Also, DTaI can competitively displace [125I]-insulin with an ED50 of 1.1 x 10(-8) M, which is identical to the ED50 of insulin (1.1 x 10(-8) M) and showed limited cross-reactivity with the IGF-1 receptor (12% displacement of [125I]-IGF-1 with a DTaI concentration of 1.1 x 10(-8) M). We have used DTaI to select conjugate-resistant clones from the V-79 Chinese hamster fibroblast parental cell line. Conjugate-resistant variants expressed insulin binding levels ranging from 8.0 +/- 2.0 fmoles/mg protein down to 3.6 +/- 0.5 fmoles/mg protein while insulin binding in the V-79 parental cell line was 11.2 +/- 0.2 fmoles/mg protein. Additionally, a number of conjugate resistant clones expressed variable ability to grow in medium containing 5% serum. The altered ability of these clones to grow in a serum-containing medium did not correlate directly with the changes observed for insulin binding. One mutant, IV-A1-j, did not grow in a serum-free defined medium containing insulin as the predominant mitogen. This IV-A1-j mutant had a lower number of insulin receptors, no change in insulin binding affinity, no change in the rate of internalization of [125I]-insulin and no apparent difference in [125I]-IGF-1 binding. Further, insulin-stimulated sugar transport was similar to that observed in the parental cell line. Based on these observations we suggest that 1) DTaI elicits its cytotoxicological effects through the insulin receptor trafficking pathway, 2) DTaI can be used to isolate cells altered at the level of insulin binding and/or action, and 3) signal transduction mechanisms responsible for mediating insulin-dependent cell growth can be pursued using mutants such as IV-A1-j.

Characterization of the D-allose-mediated regulation of sugar transport in Chinese hamster fibroblasts

Exposure to D-allose has been demonstrated to lead to decreased 2-deoxy-D-glucose (2-DG) and 3-0-methyl-D-glucose transport in the V79 Chinese hamster lung fibroblast cell line. The effect of D-allose 1) was maximal after 4 hours exposure to the cells; 2) was optimal between 2.77 and 5.55 mM D-allose; and 3) led to a decreased Vmax for 2-DG transport with no change in the transport Km value. The decrease in 2-DG transport induced by D-allose was reversible and the reversal was differentially affected by cycloheximide, being blocked by a low concentration of cycloheximide (0.05 micrograms/ml) but not a high concentration of the inhibitor (5 micrograms/ml). D-allose did not competitively inhibit the transport of 2-DG while D-glucose under similar conditions yielded a Kl for 2-DG transport inhibition of 1.7 mM. Additionally, D-allose did not affect the phosphorylation of 2-DG by hexokinase in cell-free cytosol. The data indicate that D-allose has significant lowering effects on sugar transport activity. Additionally, while the sugar itself may be the active component in sugar transport regulation, the effect is not blocked by inhibition of protein synthesis but the synthesis of a regulatory protein(s) may be involved in the return of sugar transport following D-allose removal.

Use of a genetic variant to study the hexose transport properties of human skin fibroblasts

Human skin fibroblasts from 'normal' subjects were found to possess at least two hexose transport systems. One system was responsible for the uptake of 2-deoxy-D-glucose (dGlc), D-glucose and D-galactose, whereas the other was responsible primarily for the uptake of 3-O-methyl-D-glucose (MeGlc). The transport of dGlc was the rate-limiting step in the uptake process; over 97% of the internalized dGlc was phosphorylated and the specific activity of hexokinase was several times higher than that for dGlc transport. The dGlc transport system was activated by glucose starvation, and was very sensitive to inhibition by cytochalasin B and energy uncouplers. Fibroblasts isolated from a patient with symptoms of hypoglycaemia were found to differ from their normal counterparts in the dGlc transport system. They exhibited a much higher transport affinity for dGlc, D-glucose and D-galactose, with no change in the respective transport capacity. Transport was not the rate-limiting step in dGlc uptake by these cells. Moreover, the patient's dGlc transport system was no longer sensitive to inhibition by cytochalasin B and energy uncouplers. This suggested that the intrinsic properties of the patient's dGlc transport system were altered. It should be noted that the patient's dGlc transport system could still be activated by glucose starvation. Despite the changes in the dGlc transport system, the MeGlc transport system in the patient's fibroblasts remained unaltered. The observed difference in the properties of the two hexose transport systems in the 'normal' and the patient's fibroblasts strongly suggests that the two transport systems may be coded or regulated by different genes. The present finding provides the first genetic evidence from naturally occurring fibroblasts indicating the presence of two different hexose transport systems.

Effects of phorbol, dexamethasone and starvation on 3-O-methyl-D-glucose transport by rat thymocytes. Modulation of transport by altered trans effects

Uptake of 3-O-methyl-D-glucoside (3-OMG) into thymocytes was studied to ascertain if it is modulated by endofacial hexokinase activity or by intracellular glucose. (1) The Vmax for net uptake of 3-OMG into rat thymocytes is increased by phorbol 12-myristate 13-acetate (PMA; 40 nM) or starvation for 4 h, and decreased by dexamethasone (1 microM). Starvation for 4 h abolishes the PMA-dependent increase in 3-OMG uptake; this effect is prevented by incubation in 2-deoxyglucose (2-dGlc; 1 mM). (2) Dexamethasone decreases 2-dGlc uptake, increases the rate of 2-dGlc exit and decreases accumulation of free 2-dGlc, consistent with decreased endofacial hexokinase activity. (3) 3-OMG uptake is decreased by preloading the cells with 2-dGlc or glucose, whereas preloading with 3-OMG (40 mM) increases uptake of 3-OMG. (4) The inhibitory effect of preloaded 2-dGlc or glucose on 3-OMG uptake is decreased by PMA. (5) Preloading cells with 3-OMG (40 mM) increases 2-dGlc influx in control and dexamethasone-treated cells, but not into PMA-treated cells. (6) The maximal rate of self-exchange of 3-OMG is similar in control, PMA- or dexamethasone-treated cells. These results are consistent with the following view: 3-OMG uptake is retarded by exchange with cytosolic glucose, or 2-dGlc. PMA, by increasing endofacial hexokinase activity, or starvation depletes glucose from the endofacial surface of the transporter, and hence increase 3-OMG uptake. Dexamethasone, by decreasing endofacial hexokinase activity, increases endofacial binding of glucose, and hence decreases 3-OMG uptake. Cytosolic 3-OMG competes with glucose for endofacial sites, and hence the maximal rates of exchange uptake of 3-OMG are similar in control, PMA- or dexamethasone-treated cells, as the activity of thymocyte glucose transporters is apparently unaltered.

Evidence for the autocrine inhibition of glycolysis in human fibroblasts

We investigated the mechanism by which glucose refeeding can reverse the enhancement of glycolysis caused by glucose starvation. Human fibroblasts were deprived of glucose for 18 hr and then refed for 1 hr with either (a) medium from sister glucose-starved cultures (controls), (b) fresh, glucose-containing medium (fresh medium), or (c) medium conditioned for 18 hr by glucose-fed cells (conditioned medium). Despite a lower glucose content, conditioned medium was significantly more effective at inhibiting the accumulation of radio-labeled glucose than fresh medium (74 vs. 49% inhibition). The uptake of 2-deoxyglucose was not affected by either medium, indicating that the site of control of glycolysis was distal to glucose transport and phosphorylation. The active principle was heat labile, dialyzable (Mr less than 12,000) and unrelated to the lactate content of conditioned medium. Medium conditioned by cells exposed to 3-O-methylglucose did not inhibit glycolysis in glucose-starved cells even though long-term exposure to this hexose, like glucose, results in the repression of transport.