Alterations in the characteristics of 2-deoxy D-glucose transport into cultured human glioma cells during cell growth

Glucose transport in primary cultured neurons

In this study, we have examined glucose uptake and its regulation by insulin in primary cultured neurons. Glucose transport was assessed by measuring the initial rate of uptake of 3H-2-deoxyglucose, a glucose analog that is transported and phosphorylated but not further metabolized. The uptake of 2-deoxyglucose was saturable; measurements of the intracellular concentration of 2-deoxyglucose and 2-deoxyglucose-6-phosphate revealed that hexokinase activity rather than membrane transport is the rate-limiting step for glucose uptake. Insulin had no effect on 2-deoxyglucose uptake at low (0.2 mM) or high (20 mM) concentrations of substrate. The order of potency of other hexoses to competitively inhibit the accumulation of 2-deoxyglucose was D-glucose (0.2 mM) = D-mannose (0.2 mM) greater than 3-0-methylglucose (9 mM) greater than D-galactose (90 mM). Cytochalasin B was a potent inhibitor of 2-deoxyglucose uptake (IC50 = 500 nM) and phloretin was more potent than ploridzin in inhibiting uptake. The structure of glucose transporters was examined by photoaffinity labeling using 3H-cytochalasin B and by immunologic detection using antibodies raised against the human erythrocyte transporter. 3H-cytochalasin B labeled two proteins of 55 kDa and 43 kDa and the antibody recognized primarily a 43 kDa protein. The subcellular distribution of glucose transporters, estimated by measuring the number of specific cytochalasin B binding sites in subfractions of neuronal homogenates, showed 3.62 pmol/mg protein in the 11,000g pellet and 1.34 pmol/mg protein in the 200,000g pellet.

IN CONCLUSION

1) Neuronal glucose transport is not acutely regulated by insulin. 2) The kinetics of 2-deoxyglucose uptake into neurons are determined largely by hexokinase activity rather than membrane transport. 3) The apparent molecular weight of neuronal glucose transporters is similar to transporters in other tissues. 4) The number of glucose transporters per milligram of protein is relatively low in neurons compared to other tissues.

Unidirectional influx and phosphorylation of glucose analogues in cultured astroblasts

Unidirectional influx of 14C-3OMG (3-O-methyl-D-glucose) and rate of phosphorylation of 14C-2DG (2-deoxy-D-glucose) were determined in primary cultures of astroblasts under conditions with negligible unstirred layers. The influx exhibited rate constants between 7.2 and 8.3 min-1 in the concentration range 2.5-25 mM of unlabeled 3OMG and was considered constant, irrespective of concentration of 3OMG. The rate of phosphorylation of 14C-2DG declined for rising concentrations of 2DG and hence showed saturability. The rate constants ranged from 7.9 to 0.1 min-1 in the concentration range 0.04-25 mM of 2DG. These results are not consistent with the view that the influx limits the rate of phosphorylation. They support the notion that the influx is not rate limiting for the phosphorylation.

D-Glucose transport into suspended human fibroblasts. Rapid measurement of uptake by silicone oil filtration centrifugation, and comparison of different cell detachment procedures

The uptake of 14C-labeled D-glucose into the cellular space of human diploid fibroblasts (Flow 2000)--grown to confluency and detached with trypsin-EDTA--was studied using silicone-oil-layerfiltering centrifugation. This method is rapid enough to enable the determination of initial transport rates, which are not complicated by subsequent metabolism of the hexose taken up into the cells. D-Glucose uptake shows saturation kinetics with a Km of 1.8mM and maximal transport capacity of 4--8 nmol/(10(6) cells x min) at 20 degrees C. This saturable transport system is responsible for at least 80% of the total glucose taken up into the cells in the concentration range tested (0.1--10mM D-glucose in incubation medium). The glucose carrier is stereo-specific, is independent of sodium and potassium ions, and is inhibited by cytochalasin B. Its temperature dependence reveals an activation energy of 3 1 kJ/mol (7.5 kcal/mol; Q10 approximately equal to 1.5). As detachment of the cells from the culture flasks is necessary for applying silicone-layer-filtering centrifugation, various detachment procedures were tested. In the enzymatic procedure cells were treated with either trypsin or pronase. In the chelating method, Ca2+ and Mg2+ ions were chelated by EDTA and K+ ions with sodium tetraphenylborate. For mechanical detachment, cells were grown initially on plastic foil. After each of these detachment procedures the transport of D-glucose was the same. It is therefore concluded that this method of rapid measurement of D-glucose uptake in suspended human fibroblasts may serve as an alternative to the uptake measurement with glucose analogues in attached cells when studying the hexose transport system in human diploid fibroblasts.

Endogenous cyclic AMP does not modulate transport of hexoses, nucleosides, or nucleobases in Chinese hamster ovary cells

In a previous study we have demonstrated that neither extracellular nor intracellular cyclic adenosine monophosphate (AMP) levels directly affect the uptake of nucleosides, nucleobases, or hexoses by various types of cultured mammalian cells. Uptake of these nutrients into cells, however, involves two processes operating in tandem: facilitated transport across the membrane and intracellular phosphorylation; and uptake rates generally reflect the rates of substrate phosphorylation rather than of transport. In the present study we have examined the question of whether substrate transport per se is regulated by intracellular cyclic AMP. Initially various cell lines, grown both in suspension and monolayer culture, were screened for their cyclic AMP response to prostaglandin E1, isoproterenol, and inhibitors of cyclic AMP phosphodiesterase. Prostaglandin E1 treatment of Chinese hamster ovary cells was selected as the system giving the largest and most consistent (50-fold to 100-fold) elevation of cyclic AMP. Rapid kinetic techniques were used to measure the transport of 3-O-methylglucose, thymidine, adenosine, hypoxanthine, and adenine in wild-type cells and in mutant sublines incapable of phosphorylating these substrates. In no case was an increase in intracellular cyclic AMP accompanied by a significant change in the rate of transport of these substrates, although prostaglandin E1 slightly inhibited the transport of various substrates.