Studies on the mechanism of serum stimulation of uridine uptake in serum-less mouse cells

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.

Induced alteration in uptake properties of Tetrahymena and its association with the development of mating competency

The rate of uridine uptake in Tetrahymena declines by an order of magnitude by two hours after shiftdown to a non-nutrient buffer. This alteration in uptake properties cannot be accounted for by an increase in the intracellular pool of uridine, an increase in apparent Km for uptake or a decline in the rate in which uridine is processed intracellularly. It is argued that the decrease in uridine uptake is due to a reduction in numbers of functional transport molecules exposed at the cell surface and is a reflection of a developmentally related cell surface transformation. In addition, the putative decline in functional transport molecules cannot be entirely explained by metabolic turnover of these molecules in the absence of replacement, nor does it require the synthesis of new protein. We discuss the possibility that a shift in equilibrium between accessible and inaccessible transporters is operating. Finally, a close correlation between conditions which elicit the transport alteration and those which allow the development of mating competency suggests that the two phenomena may be coordinately regulated.

Evidence for genetic control of glycine uptake in cultured cells, regulated by the amino acid concentration of the growth medium

1. Cultured cells were grown in various concentrations of amino acids for periods up to 3 days and the characteristics of the glycine transport system measured under fixed experimental conditions. During this time, the effect of enucleation, using cytochalasin B, and the effects of protein synthesis inhibitors (cycloheximide and actinomycin D) were investigated. 2. Glycine influx is regulated by the prior growth concentration of similarly transported amino acids. 3. The modification in transport involves primarily a change in Vmax (but also a change in Km in HeLa cells) and is effected within 2-10 hr after media change. Increased transport activity is calculated to be sufficient to compensate for the reduction in extracellular amino acid concentration, so that nearly normal influx values from media are maintained. Regulation over the range of concentrations studied is shown to be very accurate. 4. The nucleus is essential for the regulatory mechanism to function. It seems probable that mRNA synthesis is required for acquisition of increased transport activity and mRNA translation required for maintenance of normal activity. 5. The controlling factor in the regulatory mechanism appears unlikely to be intracellular pool size. Other possible signals are discussed.

Uridine uptake in a unicellular eukaryote during the interdivision period and after growth arrest

The rate of uridine uptake was measured in Tetrahymena after shiftdown to non-nutrient physiological salt solution. Uptake follows Michaelis-Menten kinetics and an apparent Km of transport of 2 X 10-minus 6 M has been estimated. This value is in good agreement with those reported for tissue-derived cells in culture. Incorporation of uridine into RNA follows similar kinetics suggesting that uptake is rate limiting for incorporation. Within three hours after shiftdown the rate of uptake is decreased by an order of magnitude. Also at three hours after shiftdown pairing occurs between cells of complementary mating types. It seems likely that the change in uptake is a reflection of a surface change associated with differentiation. The rate of uptake was also measured during the interdivision period using cells synchronized by a physical selection procedure. A change in rate occurs at the time the cells begin replication of DNA and is essentially stable thereafter. These results indicate that there exists in Tetrahymena a relationship between surface properties as assayed by uridine uptake and properties of growth and differentiation.

An altered rate of uridine transport in membrane vesicles isolated from growing and quiescent mouse 3T3 fibroblast cells

Balb/c 3T3 and simian virus-40-transformed Balb/c 3T3 (SV-3T3) cells were examined for their ability to transport nucleosides. Confluent (quiescent) 3T3 cells transported uridine at a rate 3-4 fold lower than did subconfluent cells. Adenosine uptake was independent of cell population density. Both adenosine and uridine were transported at the same rate by confluent and sub-confluent SV-3T3 cells. A membrane vesicle population (plasma membrane and endoplasmic reticulum) was isolated from 3T3 and SV-3T3 cells by nitrogen cavitation. With membrane vesicles derived from SV-3T3 cells it was determined that uptake occurred by a mediated process. Uptake of adenosine and uridine by membrane vesicles isolated from 3T3 and SV-3T3 cells demonstrated the same pattern as found in whole cells; that is, membrane vesicles from confluent 3T3 cells transported uridine at a rate 3-fold lower than did membrane vesicles from subconfluent 3T3 cells. Much of the adenosine taken up was converted to inosine, hypoxanthine, and ribose 1-phosphate, whereas uridine transport resulted only in the accumulation of uridine. Results obtained with membrane vesicles indicate that the lowered rate of uridine transport by confluent 3T3 cells seems due to an alteration in the membrane itself or a component(s) thereof, rather than to changes in subsequent cellular metabolic processes.

Evidence for the genetic control of the sodium pump density in HeLa cells

1. HeLa cells were grown in normal and altered growth solutions; the ion contents, volumes, K sensitive ouabain binding, the Na-K-ATPase and the Na and K transport measured.2. Cells grown in 1 x 10(-4)M ethacrynate or low-K media for 24 hr have a raised [Na](i), a decreased [K](i), and an increased ouabain binding. Those grown in low-K also have an increased Na-K-ATPase activity.3. When cells are put into low-K solutions the [Na](i) initially rises to a high value, and then starts to fall some 8 hours later as the ouabain binding increases, suggesting that these additional sites represent working Na pumps. Flux measurements on low-K cells provide some support for this view.4. Experiments in which sorbitol replaced [Na](o) showed that the increased ouabain binding and Na-K-ATPase was related to the increase in [Na](i) rather than the decrease in [K](i) and was not due to a non-specific effect of [K](o) change.5. The protein synthesis inhibitors cycloheximide and puromycin stopped the effect of ethacrynate and low-K solutions on increased ouabain binding. They also decreased the ouabain binding and K influx in normal cells over 24 hr. Cycloheximide had similar effects on Na-K-ATPase in low-K treated and normal cells. These results suggest that protein synthesis is required for the appearance of more ouabain sensitive sites in the cell membrane, both in response to ethacrynate and low-K treatment and for normal replacement during the cell's life.6. The RNA synthesis inhibitors actinomycin D (AMD) and cordycepin had complex effects on ouabain binding in fresh and ethacrynate treated cells. These inhibitors increased the ouabain binding but decreased the K influx. This discrepancy was due to the appearance of ouabain binding sites with different characteristics from normal sites. A limited investigation of this phenomenon was carried out. Probably AMD stops the normal replacement of sites in the membrane.7. These results are consistent with the hypothesis that HeLa cells have a system for controlling the number of Na pumps in their membranes. This system responds to the level of [Na](i) within the cell and involves protein synthesis. It is not clear to what extent the nucleus is normally involved in this process.

Effects of serum on membrane transport. I. Separation and preliminary characterization of factors which depress lysine or stimulate adenosine transport in rabbit alveolar macrophages

The effects of normal rabbit serum (NRS) on two transport systems in rabbit lung macrophages have been examined. A 20 min preincubation with serum was required for the effects, which were retained for at least 40 min after serum was removed. No serum was present during the transport studies. (a) Preincubation with 0.5 or 1.0% NRS resulted in depression of lysine transport to 59 +/- 2.6% (SE, 31 observations) of control levels. The activity was heat stable to 100 degrees C for 30 min and lost after dialysis. Pretreatment with serum did not alter the intracellular concentration of lysine attained when cells were then incubated with 10 mM lysine for 30 min. The relative depression of lysine transport by serum was unaltered by preloading with such high concentrations of lysine. (b) Preincubation with 5% NRS resulted in enhancement of adenosine transport by 35 +/- 2.3% (SE, 60 observations). Activity was stable to heating at 65 degrees C for 40 min but lost at 100 degrees C for 20 min. It was nondialyzable. Total radioactivity accumulated after 30 min incubation with 1 mM adenosine was unaffected by serum pretreatment. The two activities were separable by passage over Sephadex G25.