The application of rapid kinetic techniques to the transport of thymidine and 3-O-Methylglucose into Mammalian cells in suspension culture

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.

Thymidine transport in cultured mammalian cells. Kinetic analysis, temperature dependence and specificity of the transport system

The transport of thymidine has been characterized kinetically and thermodynamically in Novikoff rat hepatoma cells grown in culture and, less extensively, in mouse L cells, Chinese hamster ovary cells, P388 murine leukemia cells and HeLa cells. That the characterizations pertained to the transport system per se was ensured, (i) by employing recently developed methods for rapid sampling of cell/substrate mixtures in order to follow isotope movements within a few seconds after initial exposure of cells to substrate; (ii) by utilizing cells rendered, by genetic or chemical means, incapable of metabolizing thymidine; and (iii) by demonstrating conformity of the transport data to an integrated rate equation derived for a simple, carrier-mediated system. The results indicate that thymidine is transported into mammalian cells by a functionally symmetrical, non-concentrative system for which the carrier : substrate dissociation constant ranges from about 100 microM in Chinese hamster ovary cells, to 230 microM in Novikoff hepatoma cells. In all cell lines investigated, the velocity of transport was sufficient to nearly completely equilibrate low concentration of thymidine across the membrane membrane within 15 s. Temperature dependence of transport velocity and substrate : carrier dissociation were continuous (EA = 18.3 kcal/mol, delta H0' = 9.3 kcal/mol, respectively), and showed no evidence of abrupt transitions. Several natural and artificial nucleosides and nucleic acid bases inhibited influx of radiolabeled thymidine, apparently by competing with thymidine for the transport carrier.

The kinetic dissection of transport from metabolic trapping during substrate uptake by intact cells. Uridine uptake by quiescent and serum-activated Nil 8 hamster cells and their murine sarcoma virus-transformed counterparts

1. We present a theoretical analysis of the tandem processes of transport and metabolic trapping which together constitute uptake of a substrate by intact cells. 2. Transport is assumed to occur by means of a simple carrier here analysed in its general form. Trapping is assumed to occur by a simple enzymic reaction. 3. We show how to obtain the separate parameters of the steps by analysing uptake data over a range of uptake times and substrate concentrations. 4. We present uptake data for uridine and cytosine-beta-D-arabinoside entering Nil 8 hamster fibroblasts, normal and murine sarcoma virus transformed, in the quiescent condition and after stimulation by added serum. We analyse the data in terms of the theory for tandem processes. 5. Transport is characterised by a system having a high Km and a high V for entry. The data for cytosine-beta-D-arabinoside suggest that the cytosine-beta-D-arabinoside system is not far from a symmetric one. The data for uridine transport do not differ when quiescent and serum-activated cells are compared. Transformed cells transport uridine at half the maximum velocity of normal cells, with or without added serum. 6. Trapping of cytosine-beta-D-arabinoside is insignificant. Trapping of uridine occurs by a system with both V and Km at least an order of magnitude smaller than are these parameters for transport. Trapping of uridine by non-transformed cells activated by serum, has twice the V of such cells in the quiescent state. 7. In the virus-transformed cells, the control of uridine trapping by added serum is lost, along with control of growth by this stimulant.

Alteration of human breast tumor cell membrane functions by chromosome-mediated gene transfer

BOT-2 cells (human breast tumor origin) have an impaired ability to utilize exogenous thymidine. Previous studies revealed this deficiency to be the permeation event rather than phosphorylation, since the cells have active thymidine kinase. Chromosome-mediated gene transfer was used to transfer genetic information in the form of metaphase chromosomes, from HeLa-65 cells to the BOT-2 cells, correcting the permease deficiency. Poly-L-ornithine or lipochromes were used for facilitation of chromosome uptake. After selection on HAT medium, transferant clones were isolated at a frequency of 4 x 10(-5) and 1 x 10(-5), respectively. Transferants MGP-1 and MGL-1 are stable after 18 months and have been characterized on the bases of purine and pyrimidine nucleoside uptake, relative thymidine kinase activities, alkaline phosphatase activities, and hydrocortisone-induced alkaline phosphatase activity. MGP-1 demonstrates positive thymidine uptake and incorporates radiolabeled thymidine into DNA. MGL-1 remains thymidine transport-deficient and surveys on HAT by increasing endogenous dihydrofolate reductase activity. Alkaline phosphatase activity in MGL-1 is similar to HeLa-65, 2% of that in BOT-2, and in addition, is inducible 25-30-fold by 3 micro M hydrocortisone. We have separated, genetically, a thymidine permease function from phosphorylation in cells of human origin and have transferred genetic information for the regulation of alkaline phosphatase.

Uridine transport and phosphorylation in mouse cells in culture: effect of growth-promoting factors, cell cycle transit and oncogenic transformation

The rapid increase in uridine uptake produced by the addition of serum to quiescent cultures of fibroblasts is primarily caused by an enhanced rate of nucleoside phosphorylation. While quiescent and serum-stimulated cells display identical initial rates of transport, they show a considerable change in the composition of the acid-soluble pools labelled with [3H] uridine for five seconds. The radioactivity recovered in the phosphorylated pools increases 2-, 3-, 4- and 6-fold after addition of serum to cultures of Swiss 3T3 cells, tertiary mouse embryo fibroblasts, Swiss 3T6 and Balb 3T3, cells respectively. Furthermore, insulin, a growth factor isolated from medium conditioned by SV40 BHK cells (FDGF) and epidermal growth factor (EGF) also stimulate uridine phosphorylation within minutes. The initial rate of uridine uptake is 2- to 3-fold faster in rapidly growing normal and Simian virus 40 or polyoma virus transformed 3T3 cells as compared to untransformed 3T3 cells in the quiescent state. When quiescent cultures of 3T3 or mouse embryo cells are stimulated to leave G1 and enter into DNA synthesis, transport increases several hours after addition of serum and apparently coincides with the S phase of the cell cycle. The results demonstrate that an increase in uridine phosphorylation is a rapid metabolic response elicited by growth-promoting agents in a variety of cell types and that uridine transport and phosphorylation are independently regulated.

Properties of the thymidine transport system of Chinese hamster ovary cells as probed by nitrobenzylthioinosine

The transport of thymidine into Chinese hamster ovary cells grown in suspension culture was measured under conditions in which thymidine was not metabolized, namely, when cells had been depleted of ATP. The system transporting thymidine was saturable (Kztm = 70 micron), rapid 50% of transmembrane equilibrium level attained within 8 sec), and was apparently shared by other nucleosides, but not thymine or hypoxanthine. 6([4-nitrobenzyl]thio)-9-beta-D-ribofuranosylpurine, "nitrobenzylthioinosine", inhibited thymidine transport in a simple, noncompetitive fashion with an apparent Ki = 1.0 nM (based on total concentration of inhibitor, which significnatly overestimates that of free inhibitor). The rate of expression of inhibition was slow (t 1/2 = 17 sec) relative to the rate of association of thymidine with its transporter, and thymidine partially protected the transport system against inhibition by nitrobenzylthioinosine. The dissociation constant for the inhibitor-transporter complex was estimated at about 0.1 nM, and the number of binding sites per cell at about 6 x 10(4). HeLa, P388 murine leukemia, and mouse L cells were as sensitive to nitrobenzylthioinosine inhibition of thymidine transport as Chinese hamster ovary cells; Novikoff rat hepatoma cells were much less sensitive.

Deoxyglucose and 3-O-methylglucose transport in untreated and ATP-depleted Novikoff rat hepatoma cells. Analysis by a rapid kinetic technique, relationship to phosphorylation and effects of inhibitors

Detailed time courses of uptake of labeled 3-O-methyl-D-glucose and 2-deoxy-D-glycose by untreated and ATP-depleted Novikoff rat hepatoma cells were determined as function of concentration (0.2-10 mM) by a rapid mixing/sampling technique which allows uptake measurements in time intervals as short as 1.5 seconds. Intracellular accumulation of 3-O-methylglucose in untreated and ATP-depleted cells and of deoxyglucose in ATP-depleted cells to equilibrium followed pseudo-first order kinetics and initial velocities were computed from overall time courses of substrate accumulation. Initial velocity was a Michaelis-Menten function of exogenous substrate concentration. The estimated kinetic constants for zero-trans transport of 3-O-methylglucose were about the same for untreated and ATP-depleted cells (Kztm = 1.73 +/- 0.24 mM; Vztmax = 28.8 +/- 3.6 pmoles/microliter cell H2O. sec) and were similar to those for deoxyglucose transport in ATP-depleted cells (Kztm = 0.65 +/- 0.1 mM; Vztmax = 19.6 +/- 1.6 pmoles/microliter cell H2O. sec). Similar kinetic parameters were obtained for the transport of D-glucose and D-galactose in ATP-depleted cells. The transport of 3-O-methylglucose and deoxyglucose were inhibited by each other in a simple competitive manner with apparent Ki's similar to their transport Km's. In untreated cells, in which deoxyglucose was phosphorylated, intracellular steady-state levels of free deoxyglucose accumulated within 10 to 20 seconds of incubation regardless of its concentration in the medium. Thereafter, the rate of deoxyglucose incorporation into total cell material reflected the rate of phosphorylation rather than the transport rate. The rate of deoxyglucose transport exceeded the initial rate of its phosphorylation by 20-40 %. The intracellular steady-state-levels observed during the first 2 minutes of incubation decreased from about 40% of equilibrium level at 0.2 mM deoxyglucose to about 8% at 10 mM. Computer fits of a kinetic equation describing transport and phosphorylation as independent processes operating in tandem to these data are consistent with the observed kinetic constants for hexose transport and hexokinase activity with deoxyglucose as substrate. Upon longer incubation (2-10 minutes) the rate of deoxyglucose uptake by the phosphorylating cells decreased progressively, concomitant with a decrease in intracellular ATP and an increase in intracellular deoxyglucose to equilibrium levels. It is demonstrated that the rate of deoxyglucose uptake, measured at two or more minutes, seriously underestimates the hexose transport rate and yields misleading conclusions regarding the extent and type of inhibition by transport inhibitors, such as persantin or cytochalasin B. Persantin inhibited hexose transport in a simple non-competitive manner (Ki = 20 muM) indicating that the drug affects the function of the hexose carrier.

Uptake of thymidine into isolated rat hepatocytes. Evidence for two transport systems

Thymidine transport was studied in isolated rat hepatocytes. In these cells no phosphorylation of the substrate by thymidine kinase occurred subsequent to transport. Results from studies of the concentration-dependent uptake of thymidine indicated two transport systems with about 80-fold differences in their kinetic constants. These systems were denoted as high affinity [Km = 5.3 micron, V = 0.47 pmol/(10(6) cells X s)] and low affinity systems [Km = 480 micron, V = 37.6 pmol/(10(6) cells X s)]. From intracellular to extracellular distribution ratios of [3H]thymidine it could be concluded that the uptake by the high affinity system was a concentrative process while the transport by the low affinity system was non-concentrative. The uptake of [3H]-thymidine by the high affinity system could only be inhibited by unlabeled thymidine. In contrast, all other nucleosides tested (uridine, 2'-deoxycytidine, and 2'-deoxyguanosine) were equally effective in inhibiting the low affinity system competitively. The results would suggest that in hepatocytes lacking phosphorylation by thymidine kinase, thymidine is taken up by a high and a low affinity system working in tandem. The high affinity system seems to be an active transport process with narrow substrate specificity. Thymidine uptake by the low affinity system is a facilitated diffusion process. This system is considered to be a common transport route for nucleosides of different structures.

Sodium concentrations affect metabolite uptake and cellular metabolism

Rates of uptake of glucose (measured with 3H-2-deoxy-d-glucose), galactose, and leucine after exposure of chick embryo cells to increasing concentrations of Na+ over the range 100 to 200 mM. Uptake of nucleosides was unaffected by [Na+] over this range. Prior exposure of cells was required for the [Na+] effect on uptake. Changes were measureable within two hours after changing [Na+], and although the capacity for deoxyglucose uptake remained constant thereafter, the capacity for leucine uptake continued to change during the next few hours. Inhibition of protein synthesis by cycloheximide, or of RNA synthesis by Actinomycin D, failed to prevent these uptake changes. Analysis of the kinetics of uptake showed that only the Km for uptake of deoxyglucose or leucine was affected by [Na+]; the maximum V for each compound remained the same. Effects of [Na+]; could be distinguished from the increased capacity for glucose uptake induced by glucose starvation. Incorporation of both radioactive uridine into RNA, and radioactive thymidine into DNA, were affected by [Na+[, but the differences were not correlated with uptake of other metoblites. No differences in countable mitoses were apparent, although the growth of chick embryo cells in increased slightly with increasing [Na+]. Changes in uptake due to differing [Na+] also were observed in mammalian (rat NRK) cells. However, no effects of [Na+] on rates of cell growth or saturation density were observed with these cells.

The selection of a stable rat hepatoma variant with concomitant increase in ploidy and permeability to glycerol

By repeated selection for longer survival in an isotonic solution of glycerol, a stable subline of Novikoff rat hepatoma cells has been isolated. The cells exhibit markedly increased resistances to osmotic lysis in isotonic solutions of glycerol. They are twice as large and have twice as many chromosomes as cells of the parental line. It is suggested that the osmotic stress procedure can be extended for the selection of numerous kinds of mutants and can be used as a method of analysis of membrane properties.

Metabolic stability of the nucleoside transport system of Novikoff rat hepatoma cells

Rates of transport of uridine and thymidine, estimated with a rapid sampling technique, did not change with culture age. Inhibition of cellular RNA and protein synthesis for periods up to 6 h, did not lead to a loss of nucleoside transport activity. Mild treatment of cell suspensions with trypsin or neuraminidase had no effect on the kinetics of thymidine transport. Thus we conclude, contrary to previous reports, that nucleoside transporters are metabolically stable and that the decreases in nucleoside uptake rates observed with decreased protein synthesis reflect loss of nucleoside kinase activities. These kinases (which have narrow substrate specificity) rather than the membrane-associated, transport apparatus (which has broad substrate specificity) are the most likely sites for regulation of nucleoside uptake.

Suppression of phytohemagglutinin-induction of thymidine uptake in guinea pig lymphocytes by methylglyoxal bis(guanylhydrazone) treatment

Treatment with methylglyoxal bis(guanylhydrazone), a specific inhibitor of S-adenosylmethionine decarboxylase (EC 4.1.1.50), suppressed the phytohemagglutinin-induction of [3H]thymidine uptake by guinea pig lymphocytes. The kinetics of [3H]thymidine uptake revealed that the Km value for thymidine was not changed, but the V value was markedly lowered by the methylglyoxal bis(guanylhydrazone) treatment. The induction of ATP: thymidine 5'-phosphotransferase (EC 2.7.1.75) (thymidine kinase) activity by phytohemagglutinin was suppressed to about the same extent as the induction of thymidine uptake. These suppressions were dependent on the methylglyoxal bis(guanylhydrazone) doses and on duration of the methylglyoxal bis(guanylhydrazone) treatment. Analysis of [3H]thymidine labelled compounds of the acid-soluble fraction showed that conversion of thymidine to thymidine 5'-triphosphate was inhibited by the methylglyoxal bis(guanylhydrazone) treatment. DNA polymerase activity was less inhibited by the methylglyoxal bis(guanylhydrazone) treatment in comparison with the methylglyoxal bis(guanylhydrazone) inhibition of thymidine uptake by whole cells. These results strongly suggested that blocking of polyamine accumulation by the methylglyoxal bis(guanylhydrazone) treatment influenced phytohemagglutinin induction of thymidine phosphorylation, resulting in a decrease of thymidine incorporation into DNA.

Growth rate of cultured Novikoff rat hepatoma cells as a function of the rate of thymidine and hypoxanthine transport

Novikoff rat hepatoma cells were propagated in suspension culture in the presence of 1 micron methotrexate and various concentrations of hypoxanthine (or adenosine plus guanosine) and thymidine and with or without the inhibitor of nucleoside and purine transport, Persantin (dipyridamole). Methotrexate-treated cells failed to replicate and died even if the medium was supplemented with either thymidine or a purine source, but normal replication occurred when both were present. The additional presence of Persantin reduced the rate of transport of thymidine or hypoxanthine and thus their incorporation into the nucleotide pool and decreased the rate of cell replication. The growth rate of the cells was directly proportional to the rate of incorporation of thymidine (in the presence of excess hypoxanthine) or of hypoxanthine (in the presence of excess thymidine) until the normal maximum growth rate was obtained. Normal cell replication in the presence of methotrexate and Persantin occurred only when the medium was supplemented with 500 micron hypoxanthine and 30 micron thymidine. The results illustrate a dependence of the growth rate of mammalian cells on the rate of transport of essential nutrients into the cell.

Relationship between thymidine transport and phosphorylation in Novikoff rat hepatoma cells as analyzed by a rapid sampling technique

Incorporation of thymidine into Novikoff rat hepatoma cells was analyzed with a rapid sampling technique which allowed collection of 12 time points in 20 sec. Transport was a rapid, saturable, nonconcentrative process with a Km of about 85 micrometer. The intracellular thymidine pool was also rapidly labeled in cells which phosphorylated thymidine, that a group translocation process involving thymidine kinase can be ruled out. Under all conditions examined, phosphorylation, not the transport, of thymidine was the rate-determining step in its incorporation into the acid-soluble pool. Estimation of transport rates from total incorporation into cells which phosphorylate the substrate is invalid in this cell system and must be questioned in all instances.