Regulation of amino acid transport activity and growth rate of animal cells in culture

Kinetics of precursor labeling in stable isotope labeling in cell cultures (SILAC) experiments

Recent advances in mass spectrometry have enabled proteome-wide analyses of cellular protein turnover. These studies have been greatly propelled by the development of stable isotope labeling in cell cultures (SILAC), a set of standardized protocols, reagents aimed at quantifying the incorporation of (15)N/(13)C labeled amino acids into proteins. In dynamic SILAC experiments, the degree of isotope incorporation in proteins is measured over time and used to determine turnover kinetics. However, the kinetics of isotope incorporation in proteins can potentially be influenced not only by their intracellular turnover but also by amino acid uptake, recycling and aminoacyl-tRNA synthesis. To assess the influence of these processes in dynamic SILAC experiments, we have measured the kinetics of isotopic enrichment within intracellular free amino acid and aminoacyl-tRNA precursor pools in dividing and division-arrested neuroblastoma cells following the introduction of extracellular (15)N labeled amino acids. We show that the total flux of extracellular amino acids into cells greatly exceeds that of intracellular amino acid recycling and synthesis. Furthermore, in comparison to internal sources, external amino acids are preferentially utilized as substrates for aminoacyl-tRNA precursors for protein synthesis. As a result, in dynamic SILAC experiments conducted in culture, the aminoacyl-tRNA precursor pool is near completely labeled in a few hours and protein turnover is the limiting factor in establishing the labeling kinetics of most proteins.

Cell suicide in starving hybridoma culture: survival-signal effect of some amino acids

Two mouse hybridoma cell lines cultured in different basal media with the iron-rich protein-free supplement were subjected to deliberate starvation by inoculation into media diluted with saline to 50% or less. In the diluted media the growth was markedly suppressed and a large fraction of cells died by apoptosis. The cells could be rescued from apoptotic death by individual additions of amino acids, such as glycine, L-alanine, L-serine, L-threonine, L-proline, L-asparagine, L-glutamine, L-histidine, D-serine, β-alanine or taurine. Amino acids with hydrophobic or charged side chains were without effect. The apoptosis preventing activity manifested itself even in extremely diluted media, down to 10% of the standard medium. The activity of L-alanine in the protection of cells starving in 20% medium was shown also in semicontinuous culture. In the presence of 2 mM L-alanine the steady-state viable cell density more than doubled, with respect to control, and the apoptotic index dropped from 37% in the control to 16%. It was concluded that the apoptosis-preventing amino acids acted as signal molecules, rather than nutrients, and that the signal had a character of a survival factor. The specificity of present results, obtained with two different hybridomas, supports our view (Franěk and Chládková-Šrámková, 1995) that the membrane transport macromolecules themselves may play the role of the recognition elements in a signal transduction pathway controlling the survival of hybridoma cells.

Cell density inversely regulates D- and L-aspartate levels in rat pheochromocytoma MPT1 cells

In a previous report (FEBS Lett. 434 (1998) 231), we demonstrated for the first time that D-aspartate (D-Asp) is synthesized in rat pheochromocytoma 12 (PC12) cells. This unique amino acid is believed to act as a novel messenger in mammalian cell regulation. However, the dynamics of D-Asp homeostasis in mammalian cells is yet to be elucidated. In this communication, we demonstrate that D-Asp is also synthesized in MPT1 cells (a subclone of PC12 cells) and that the D- and L-Asp levels in cells are regulated by cell density of the culture. Our data show that D-Asp levels increase, while in contrast, L-Asp levels decrease as a function of increased cell density. Conversely, in PC12 cells, which do not express the glutamate transporter involved in the incorporation of D- and L-Asp into cells, L-Asp levels decrease upon cell density increase while D-Asp concentrations remain almost unchanged. The results indicate that the biochemical behaviors of D- and L-Asp in mammalian cells are distinct and that the cellular levels of these stereoisomers appear to be under different control mechanisms.

Calmodulin-dependent modulation of pH sensitivity of the amino acid transport system L in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells express the amino acid transport system L. The activity of this system is Na(+)-independent and is stimulated by acidic extracellular pH. Treatment of cells with the calmodulin antagonist CGS 9343B results in a marked stimulation of the system L activity. At a CGS 9343B concentration of 50 microM, the stimulation of activity measured at pH 7.5 is about 75-100%. This effect is not blocked by cycloheximide, actinomycin D, colchicine or cytochalasin D suggesting that the stimulation is not due to de novo synthesis of the carrier protein or recruitment of the carrier protein from an intracellular pool. The stimulatory effect of CGS 9343B is reproducible with other calmodulin antagonists. Treatment with CGS 9343B significantly modifies pH sensitivity of the system. The stimulatory effect of H+ is markedly reduced in treated cells compared to control cells. The stimulation of activity at pH 5.5 vs. pH 7.5 is 55% in control cells but only 8% in treated cells. Similarly, the stimulatory effect of CGS 9343B is reduced by H+. The stimulation of activity seen with 50 microM CGS 9343B is 80% at pH 8.0, but only 26% at pH 5.5. In addition, H+ and CGS 9343B affect the kinetic parameters of system L in a similar manner, the stimulation in both cases being primarily due to an increase in the maximal velocity. The apparent competitive nature between the effects of H+ and CGS 9343B is also observed with other calmodulin antagonists. These results show that the transport function and pH sensitivity of the amino acid transport system L in placental choriocarcinoma cells are modulated by calmodulin by processes which do not involve de novo synthesis nor recruitment of the carrier protein.

Monitoring hybridoma metabolism in continuous suspension culture at the intracellular level

A model mouse hybridoma cell line was grown in continuous culture experiments in a serum-free low-protein lipid-free medium. The steady-state responses of cell numbers, extra- and intracellular metabolite concentrations, substrate and (by) product consumption/production rates, and yield coefficients were investigated as a function of step changes in the glutamine concentration of the feed medium. In addition to the commonly performed analysis of metabolites in culture supernatants, we prepared perchloric acid extracts of cells and determined the amount and the composition of intracellular amino acids and organic acids. Significant differences were found with respect to intracellular metabolite pools for cells growing at nearly identical specific growth rates. To our knowledge this is the first time that data on the intracellular concentrations (pools) of amino acids and Krebs cycle intermediates are reported in the literature that were obtained under carefully defined culture conditions such as those attained in continuous culture experiments.

Changes in alanine transport in plasma membrane vesicles from rat liver during the early stages of diethylnitrosamine-induced hepatocarcinogenesis

The transport of L-alanine, a natural substrate of system A, across liver plasma membrane vesicle preparations was modified during the early stages of rat DENA hepatocarcinogenesis. Kinetic studies indicated an increase of the Vmax, with normal Km values, at 30 h in rats undergoing a partial hepatectomy. Normal Vmax and drastically reduced Km values were present using membrane preparations from liver tissue showing enzyme-altered hyperplastic foci and/or preneoplastic nodules. The results suggest that alanine transport is differently affected by initiating and promoting stimuli during rat DENA hepatocarcinogenesis. The changes of the Vmax could be related to the promoting effect of partial hepatectomy on cell proliferation whereas the changes of the affinity constant (Km) could be the result of intrinsic modifications of the transporter in initiated cells.

Amino acid uptake in plasma membrane vesicles isolated from proliferating tumor cells and tissues

The transport of L-alanine, a natural substrate of system A, across plasma membrane vesicle preparations has been studied in the early stages of rat DENA-PH hepato-carcinogenesis and in a very undifferentiated rat ascites hepatoma cell line (Yoshida AH-130) in the exponential and stationary phase of growth.Kinetic analyses indicated an increase of the Vmax value in DENA-PH-treated rats 30 h after partial hepatectomy as well as in exponential growing Yoshida ascites cells. In DENA-PH-treated rats the Km value was drastically reduced 7 and 60 days after surgery, when enzyme-altered hyperplastic and preneoplastic lesions were present in rat liver. Drastically reduced Km values were also found in Yoshida ascites cells.The results suggest that an altered alanine transporter might take place in liver plasma membranes from carcinogen-treated rats. This appears to occur also in an established tumor cell line, grown in vivo.

Differential adaptive response to hyperosmolarity of 3T3 and transformed SV3T3 cells

Both 3T3 and simian virus 40-transformed 3T3 (SV3T3) cells were used to investigate differences in population kinetics, protein synthesis, monovalent ion levels, and amino acid accumulations between normal and transformed cells exposed to hyperosmolarity at 0.5 Osm. Under similar culture conditions, SV3T3 cells were found to be more sensitive in their proliferative response than normal cells to the hyperosmolar treatment. In the normal 3T3 cells, the increase in transport of amino acids was less sustained and was associated with higher levels of accumulated amino acids. The equilibrium distribution of intracellular monovalent cations and the rate of protein synthesis also returned faster to baseline values in the normal cells than in the transformed cells. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis revealed the induction of a 69-kDa polypeptide in the 3T3 cells but not in the SV3T3 cells after exposure to hyperosmolarity. On electrofocusing and relative mass analysis, this polypeptide closely migrated with the 70-kDa heat shock protein (hsp) family, although it was unrelated immunologically to the inducible 72-kDa hsp.

Control of protein synthesis by extracellular Na+ in cultured fibroblasts

In chick embryo fibroblasts (CEFs), a partial substitution of extracellular Na+ with other cations or carbohydrates decreased the intracellular Na+ content without altering the K+ level. Concomitantly, a significant decrease in the serum-dependent rate of protein synthesis occurred. This phenomenon appeared to be quickly reversible upon reconstitution of the correct extracellular Na+ concentration in the culture medium. The presence of a transcriptional inhibitor such as actinomycin D during the treatment did not inhibit the reversibility of the phenomenon. The presence in the culture medium of K+ in such excess as to dissipate the membrane potential did not alter the observed relationship between the protein synthesis rate and the internal Na+ content. Analysis of the amino acid pool indicated that the observed inhibition of the rate of protein synthesis in CEFs incubated in low Na+ medium was not caused by an unbalanced availability of intracellular amino acids. In addition, intracellular pH, as estimated by the measurement of the equilibrium distribution of benzoic acid, did not show any significant alteration in cells incubated in the presence of bicarbonate buffer and in low extracellular Na+. Moreover, the relationship between the rate of protein synthesis and the internal Na+ content was still observed in CEFs cultured in bicarbonate-containing media, but at lower or higher than physiological pH. Analysis by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) of the proteins synthesized by CEFs cultured at a reduced extracellular Na+ concentration showed that specific alterations of gene expression occurred.

Growth-dependent regulation of system A in SV40-transformed fetal rat hepatocytes

Fetal RLA209-15 hepatocytes, transformed with a temperature-sensitive SV40 mutant, behave like fully differentiated cells at the growth-restrictive temperature of 40 degrees C. Conversely, incubation at the growth-permissive temperature of 33 degrees C results in a transformed phenotype characterized by rapid cell division and decreased production of liver-specific proteins. The results presented here demonstrate that the cells at 33 degrees C exhibited high rates of system A transport, but transfer to 40 degrees C reduced the activity greater than 50% within 24 h. This decline in transport was independent of cell density, although the basal rate of uptake was inversely proportional to cell density in rapidly dividing cells. Transfer of cells from 40 to 33 degrees C resulted in an enhancement of system A activity that was blocked by tunicamycin. Plasma membrane vesicles from cells maintained at either 33 or 40 degrees C retained uptake rates proportional to those in the intact cells; this difference in transport activity could also be demonstrated after detergent solubilization and reconstitution. Collectively, these data indicate that de novo synthesis of the system A carrier is regulated in conjunction with temperature-dependent cell growth in RLA209-15 hepatocytes.

Effect of L-thioproline on the progression and events of the cell cycle of HeLa cells

L-thioproline (thiazolidine-4-carboxylic acid, L-Tp) reduces DNA synthesis in cultured HeLa cells by about 50% when added to synchronized cultures at the beginning of G1 phase. This inhibition is not observed when the drug is added at mid-S, in G2 or in M. L-Tp almost annuls the burst in 22Na uptake occurring at the beginning of G1 and is as potent an inhibitor as amiloride. It is also able to delay the apparition of the early S-specific cAMP peak and to elevate its intracellular concentration. These results indicate that L-Tp could decrease tumor cell growth by acting at the G1-S boundary.

Stimulation of leucine transport by a mitogen through intracellular Ca2+ increase in human peripheral lymphocytes

The effect of concanavalin A and ionophore A23187 on leucine uptake by human peripheral lymphocytes has been examined. Preincubation of the cells with 32 micrograms/ml concanavalin A or 0.1 microM A23187 increased leucine uptake by 67% and 100%, respectively. Both concanavalin A and A23187 could, within 2 min, induce a more than 2-fold increase in the cytoplasmic free Ca2+ concentration ([Ca2+]i). This increase by concanavalin A was completely blocked by the addition of 0.1 mM 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) to incubation medium; TMB-8 partially blocked the action of A23187. The stimulation of leucine uptake by concanavalin A and A23187 was strongly inhibited by the presence of TMB-8 in the medium, whereas the basal uptake was not affected by this intracellular Ca2+ antagonist. Amiloride did not inhibit the stimulation of leucine uptake by concanavalin A. The concanavalin A- and A23187-induced elevation of [Ca2+]i was accompanied by membrane hyperpolarization. Concanavalin A-stimulated leucine uptake was greatly inhibited by the presence of an excess of 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid. These results indicate that the increase in [Ca2+]i may function as a signal of the stimulation by mitogen of leucine uptake mediated by system L, finally inducing membrane hyperpolarization in human lymphocyte.

Modulation of cellular heat sensitivity by specific amino acids

When either plateau-phase or exponentially growing Chinese hamster ovary (CHO) cells are incubated in amino acid-free medium, the cells become sensitized to killing by heat. For cells deprived of amino acids for 12 h survival decreases from 1 X 10(-2) for controls to 1 X 10(-6) for the deprived cells, following heating at 45 degrees C for 38 min. The survival of these sensitized cells is rapidly increased by the addition of a single amino acid just prior to heating. Of the 21 amino acids which are added in purified form to make McCoy's 5a medium, 12 show no protective effect, four have a small protective effect, and either alanine, asparagine, glutamine, serine, or theronine raise survival to a level similar to that of the control cells. The nonmetabolizable alanine analogue, 2-aminoisobutyric acid (AIB), increases survival of amino acid-deprived cells as effectively as each member of the group of five listed above, suggesting that metabolic conversion of the amino acids is not required for their protective effect. The data suggest that an increase in the intracellular concentrations of specific amino acids, independent of any change in cellular ATP content or the rate of protein synthesis, enables these cells to become quickly more resistant to killing by heat. We also conclude that the amino acid concentrations in poorly vascularized regions of some tumors should be considered, along with the oxygen, glucose, and proton concentrations, as factors which determine cellular survival following hyperthermia.

Protein synthesis in 3T3 cells stimulated to proliferate at various rates

Reproducible conditions were defined for using rates of leucine incorporation as a valid measure of rates of de novo protein synthesis in mouse 3T3 cells. Upon stimulation of quiescent cultures, rates of de novo synthesis of proteins increased and pool levels of amino acids decreased in proportion to the concentration of serum in the stimulating medium. Rates of de novo protein synthesis (per cell) exhibited a biphasic pattern of increase. These rates approached a plateau value at the end of the lag phase and increased again as cells entered S phase. This pattern of behaviour helps to explain the observed relationships between cell growth (increase in mass) and cell proliferation (increase in cell number).

The effect of transport system A and N amino acids and of nerve and epidermal growth factors on the induction of ornithine decarboxylase activity

The induction of ornithine decarboxylase (EC 4.1.1.17) (ODC) by amino acids and by the peptide hormones nerve growth factor (NGF) and epidermal growth factor (EGF) in salts-glucose media has been studied. Only those neutral amino acids taken into the cell via one of the Na+ dependent transport systems stimulate ODC activity. Asparagine and the nonmetabolizable alpha-amino-isobutyric acid (AIB) were used as representatives of this class of inducing amino acids, and their intracellular concentrations were related to the levels of ODC induced. A threshold intracellular concentration of asparagine or AIB has to be attained before ODC can be induced. Further slight increases in intracellular concentrations of asparagine or AIB produce disproportionately large increases of ODC, resulting in a sigmoidal curve of ODC induction. These results, and the fact that the decrease in ODC levels caused by valine is associated with a concurrent decrease in the intracellular level of the inducing amino acid, suggest that the intracellular amino acid level is causally related to the induction of ornithine decarboxylase. Glutamic acid, EGF, and NGF do not induce ODC except in the presence of an inducing amino acid. They act synergistically with the inducing amino acid and produce higher ODC levels at the same intracellular concentration of the inducing amino acid.

Adaptive regulation of neutral amino acid transport System A in rat H4 hepatoma cells

Substrate regulation of System A transport activity in rat H4 hepatoma cells is described. The uptake of several amino acids was tested in the presence of system-specific inhibitors. System A activity was increased in a RNA- and protein synthesis-dependent manner by amino acid deprivation of the cells (adaptive regulation), whereas transport by Systems ASC, N, y+, and L was unaffected. Unlike human fibroblasts, the H4 cells did not require serum to exhibit the depression of System A. At cell densities between 88 X 10(3) and 180 X 10(3) cells/cm2, the degree of adaptive regulation was inversely related to cell density. Both transport of AIB and adaptive regulation of System A were nearly abolished if either K+ or Li+ was substituted for Na+ in the medium. The presence of cycloheximide or tunicamycin blocked further increases in starvation-induced activity within 1 hr of addition, suggesting the involvement of a plasma membrane glycoprotein. In contrast, if the medium was supplemented with actinomycin after the stimulation of System A had begun, the activity continued to increase for an additional 2 hr before being slowed by the inhibitor. The contributions of trans-inhibition and repression to the amino acid-induced decay of System A activity were estimated for several representative amino acids. In general, the System A activity in normal rat hepatocytes was much less sensitive to trans-inhibition than the corresponding activity in H4 hepatoma cells. The half-life values for the amino acid-dependent decay of System A ranged from 0.5 to 2.0 hr.

Effect of theophylline on the transport of tyrosine in cultured B-16 mouse melanoma cells

The effect of theophylline, an inhibitor of phosphodiesterase, on tyrosine (Tyr) transport across cell membrane was studied using cultured B-16 mouse melanoma cells. 1.5 mM Theophylline in culture medium increased Tyr uptake velocity in linear fashion up to 36 h, after which it reached a plateau at which the cells showed about a 60% increase in Tyr transport velocity. This increase was independent of extracellular Na, essentially unaffected by cell density, and partially inhibited by concomitantly added cycloheximide. These results suggested that biosynthesis of macromolecules, probably acting as System L transporter, was induced by theophylline treatment.

Intracellular protein degradation: basis of a self-regulating mechanism for the proteolysis of endogenous proteins

The intracellular basal proteolysis system, as distinct from the lysosomal system, is important in sustaining a high flux of proteins required for maintenance, growth and adaptability of cells. Its activity automatically fluctuates with changes in protein synthetic activity, but with a considerably slower response time, since the two processes are only indirectly or passively linked. Since as much as one-third of intracellular proteolysis in mammalian cells is directed as nascent proteins, the consequences are more fully discussed in relation to cell growth state. During rapid growth, cells have to accumulate more than double their original protein mass in order to achieve a 100% increase between divisions. The effects of reducing protein synthesis by inducing quiescence, serum step-down or cycloheximide treatment on intracellular proteolysis are considered, and the possibility that this leads to enhanced degradation of existing proteins has been explored. No substantial evidence was found to support this latter notion. The basal proteolysis system is seen as a constitutive, pervasive and broad-spectrumed collection of hydrolytic enzymes. It destroys proteins randomly, having no means of distinguishing young from old, aberrant from normal. The rate of demise of protein substrates depends on two factors, the ease of access of the hydrolytic enzymes to their peptide bonds, and the length of time that any species of protein remains at risk to this hydrolytic potential. While the former has long been recognized, the importance of the second factor in relation to the ability of proteins to become integrated in the living fabric of the cell is only beginning to be appreciated. The discussion also suggests elaborate regulatory mechanisms akin to those for protein synthesis would be unnecessary for protein degradation, especially if it can now be substantiated that substrate availability determines the turnover rates of proteins by a pervasive and relatively unlimited proteolytic system (Grisolía, 1964).

Stimulatory effect of platelet homogenate on amino acid transport into L-929 cells

It was found that the addition of platelet homogenate to confluent cultures of L-929 cells causes distinct increase of leucine and proline transport into the cells. A significant part of these amino acids is incorporated into high molecular, nondialysable material. The addition of cycloheximide to culture medium completely inhibits incorporation of proline and leucine into proteins but it does not affect the transport of these amino acids into the cells.

Influence of retinoic acid on protein synthesis and transport of L-methionine in cultured L cells

The effects of retinoic acid (RA) on cell proliferation, activity of acid phosphatase, protein synthesis and methionine uptake were studied in transformed murine LPA cells. Early inhibition of protein synthesis was demonstrated under experimental conditions in which the rate of cell proliferation was diminished and non-specific effects of vitamin action could be excluded. Measurements of L-methionine uptake revealed a decrease to approximately one-half of that in control cultures after treatment with RA at the concentrations of 5 X 10(-5) M and 10(-4) M.

Influence of amphotericin B on leucine uptake in 3T3 cells

By studying the effect of leucine competitors we found that activation of the specific leucine-transport system underlies the enhancement of leucine uptake in mouse 3T3 fibroblast cells induced by sublethal doses of Amphotericin B (synergic effect). The relation of the antibiotic activity and the alteration of the membrane cholesterol interaction with lipids is discussed.

Growth-dependent AIB and meAIB uptake in LLC-PK1 cells: effects of differentiation inducers and of TPA

Cultured pig kidney cells designated LLC-PK1, previously shown to acquire Na+-dependent concentrative transport of hexoses as the cells become growth arrested, also show Na+-dependent concentrative uptake of the amino acid analogs alpha-aminoisobutyric acid (AIB) and (methyl) meAIB. This A system-like transport is most active in sparse, growing cultures and becomes stepped down at confluence. The cell/medium equilibrium distribution ratio of the lipophilic cation tetraphenylphosphonium ion (TPP+) decreases in parallel fashion, suggesting that a decrease in membrane potential may be a major factor in the stepdown. Differentiation inducers (hexamethylene bisacetamide) and phosphodiesterase inhibitors (theophylline, methylisobutyl xanthine) accelerate the stepdown, but even in the presence of these compounds addition of the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) results in the maintenance of a high level of AIB and meAIB uptake. In all these respects the changes in A system-like amino acid transport are the reciprocal of those seen for concentrative hexose transport, although the driving force appears to be the same for both systems. The TPA analogs phorbol and 4-0-methyl TPA which are inactive in tumor promotion are inactive in this system as well. In confluent, already stepped-down cultures, addition of TPA leads to a rapid (2-6 hour) stimulation of AIB and meAIB uptake. The enhancement is sensitive to cycloheximide and actinomycin D. The ouabain-sensitive fraction of meAIB uptake is not markedly changed in the TPA-enhanced uptake, nor is the TPP+ distribution ratio elevated in TPA-treated cells, making it unlikely that the TPA effect is through an alteration in the membrane potential.

The regulation by cell density of amino acid transport system L in SV40 3T3 cells

The rate of transport of phenylalanine by System L has been measured in SV40 3T3 cells at various cell densities. When the activity of the L system was determined before any cell depletion of intracellular amino acids, a density-dependent increase in transport paralleled the decrease in cell density. This regulation was lost after cell depletion but reappeared after reloading the cells with pertinent substrates of System L. The phenylalanine transport activity modulated by cell density appeared to be related to the internal level of amino acids capable of exchange up to a definite concentration, beyond which transport activity by System L did not parallel a further increase of internal substrate level. Analysis of the relationship between influx and substrate concentration suggested that two saturable components contribute to entry of phenylalanine and leucine in depleted and in reloaded cells: a low-affinity and a high-affinity component. Both kinetic parameters of the high-affinity component appeared to be modulated by the loading treatment, but only V changed markedly. Activation energies for the high-affinity component of the amino acid transport reaction were calculated from an Arrhenius plot in reloaded cells, and were found to be different for low- and high-density cultures. This result is consistent with the interpretation that cell density modulated the rates at which the amino acid-carrier complex can move within the cell membrane.

Isolation of a spontaneous CHO amino acid transport mutant by a combination of tritium suicide and replica plating

A spontaneous transport mutant of Chinese hamster ovary cells, CHY-1, was isolated by a combination of [3H]proline suicide and replica plating. The mutant took up less tritium than the parent, resulting in a lower killing rate during storage. Transport by four separate amino acid transport systems (A, ASC, L, Ly+) was examined. The CHY-1 mutant exhibited normal uptake via the ASC, L, and Ly+ systems. By contrast, uptake of the most specific substrate of the A system, 2-(methylamino)-isobutyric acid, was significantly reduced at low, but not high, concentrations, due to a 3.5-fold increase in Km and a 1.5-fold increase in Vmax. Taken together, these data suggest that the CHY-1 mutation may be in the structural gene coding for the A transport protein. The tritium suicide procedure is discussed, and general equations are derived to predict the maximum storage time for the survival of one mutant cell and the optimum size of the cell population for maximum mutant enrichment.

Membrane transport properties differ following return of serum-deprived versus Ca++-deprived human fibroblasts to a proliferative state

Human lung fibroblasts (W138) can be brought to a quiescent state by removal of serum from the medium or by lowering of the extracellular Ca++. Upon return of Ca++ or serum, the cells enter the G1 phase and progress to S within 15-18 hours. Since multiple G1 phase blocks have been demonstrated, we wished to determine whether the Ca++ and serum block were equivalent since previous data suggested that these two medium components may act at a common point in the initiation of proliferation. We have evaluated the membrane transport of 86Rb, 3-O-methylglucose, AIB, and cycloleucine following stimulation of quiescent cells by Ca++ or serum. Serum stimulation results in large increases in the influx of all the substances tested. These increases are prevented if Ca++ is absent upon serum stimulation or they are rapidly diminished following Ca++ removal. In contrast, Ca++ stimulation of Ca++-deprived cells causes little or no enhancement of any of the transport system, yet the cells progress to S phase in a manner similar to serum-stimulated cells. These results indicate that the Ca++ and serum G0 and G1 block are not equivalent and that the serum-induced change in transport of these components does not appear necessary for successful G1 phase progression. Furthermore, the data suggest that the sequence in which Ca++ or serum are presented to the cells alters the ability of Ca++ to modulate the transport systems. Quiescent cells which are exposed to Ca++ prior to serum possess a Ca++ modulation of several transport systems. Cells which are exposed to Ca++ subsequent to serum do not appear to possess this Ca++ regulation.

Influence of serum and amino acids on the accumulation of aminoisobutyrate by rat hepatoma cells. A dedifferentiation of transport routes?

Rat hepatoma cells accumulate considerably less 2-aminoisobutyrate after cultivating in the absence of serum--the change in rate of aminoisobutyrate uptake takes place within 1 h of serum starvation. Starvation of amino acids by contrast raises aminoisobutyrate uptake in the presence or absence of serum, but the cells are much less responsive to amino acid supply than to availability of serum. Phosphate (10 mM) reduced aminoisobutyrate uptake by cells grown in serum to that exhibited by serum-starved cells. Aminoisobutyrate uptake by cells grown in serum was reduced by glycine, proline, alanine, serine, glutamine, methylaminoisobutyrate and 2-aminonorbornane-2-carboxylate, the effects of methylaminoisobutyrate and 2-aminonorbornane-2-carboxylate being additive. However, similar inhibition phenomena were not seen for cells deprived of serum where aminoisobutyrate uptake tended to a relatively constant level insensitive to inhibitory influences, yet substantially greater than that arising by simple diffusion. The comparative insensitivity of our hepatoma line when starved of serum to competition and repression phenomena is in contrast to findings of others. Our results also suggest a lack of clear delineation of specificities for the A and L transport systems as usually defined.

Neutral amino acid transport during the cell cycle of cultured human lymphocytes

Neutral amino acid (isoleucine, leucine, and valine) transport was monitored in synchronous populations of WIL-2 cells, a diploid human lymphoblastoid B cell line. Cells were synchronized by either the double thymidine block technique or by density-dependent arrest in G1. Cells synchronized by these methods showed up to a 66% decrease in amino acid transport upon progressing from the early S phase through the G2 and M phases with a concomitant increase in transport during the G1 phase. The lowest activities of amino acid transport in synchronized cells were observed during the G2 and M phases.

Sulfhydryl group involvement in the modulation of neutral amino acid transport in thymocyte membrane vesicles

Membrane vesicles from rat thymocytes accumulate 2-aminoisobutyric acid in the presence of 0. 1 M NaCl. Uptake is half maximal between 15 and 30 seconds after addition of the amino acid and reaches a plateau value after about 2 minutes. The uptake of 2-aminoisobutyric acid can be modulated by various sulfhydryl reagents. Reduced glutathione leads to an inhibition of uptake whereas oxidized glutathione increases uptake. Agents such as insulin and diamide which can induce disulfide formation lead to an activation of transport. These date indicate that uptake of the Na+-dependent amino acid, 2-aminoisobutyric acid, in thymocytes is modulated by a putative plasma membrane, sulfhydryl-containing protein.

Influence of proliferative rates and A system substrate availability on proline transport in primary cell cultures of the R3230AC mammary tumor

Regulation of A system amino acid transport was studied in primary cultures of the R3230AC mammary adenocarcinoma. Higher rates of carrier-mediated Na+-dependent proline transport, vc, was decreased and was attributed to a two-fold decrease in Vmax and a two-fold increase in Km. When compared to cells grown in standard media (Eagle's minimal essential medium, MEM), cells grown in media supplemented with A system substrates (alanine, serine, glycine, and proline) demonstrated adaptive decreases in proline transport; the decrease was due to two-fold reduction in Vmax, with no change in Km for proline. Even in the presence of preferred substrates for the A system, a density-dependent decrease in proline transport was manifested. Both fast- and slow-growing cultures maintained in MEM exhibited rapid increases in proline transport when switched to buffers devoid of amino acids; two-fold increases in Vmax were seen within 4 hr, but Km was unchanged. This starvation-induced adaptation was completely prevented by inclusion in the buffer of 10 mM proline, 0.1 mM alpha-(methylamino)-isobutyric acid (MetAIB) or 10 mM serine, whereas inclusion of the poorer A system substrate, phenylalanine (10 mM), had no effect. The effects of MetAIB to prevent starvation-induced increases in proline transport were dose-related, rapid, and reversible. Amino acid starvation-induced increases in proline transport were partially blocked by cycloheximide or actinomycin D. Data were obtained demonstrating a temporal relationship between increasing intracellular [proline] and decreasing vc for proline uptake. In addition, efflux of proline from preloaded cells preceded the increase in initial rates of proline entry. Taken together, we concluded that: 1) A system transport in primary cultures of this mammary adenocarcinoma is regulated by cell density as well as by availability of A system substrates, but these two types of regulation are kinetically distinct; and 2) starvation-induced enhancement of proline transport appears to be due to release from transinhibition, but may also involve a derepression-repression type of mechanism.

Alanine transport by Chinese hamster ovary cells with altered phospholipid acyl chain composition

The Na+-dependent transport of alanine has been examined in Chinese hamster ovary (CHO) cells as a function of the fatty acid composition of their membrane lipids. Significant changes in the fatty acid composition of the CHO cell phospholipids were achieved by supplementation of the growth medium with specific saturated (palmitate) or monoenoic (oleate) free fatty acids. Arrhenius plots of the temperature-dependent uptake of alanine were constructed for cells of altered fatty acid composition. Alanine uptake was characterized by a single discontinuity in the Arrhenius plot. The temperature of this break was observed to be dependent upon the fatty acid composition of the cell phospholipids, ranging from 16 degrees C for cells enriched with oleate to 32 degrees C for cells enriched in palmitate. Calculation of the Km value for the uptake process showed no significant change with temperature or fatty acid supplementation. Correlations are made between the physical state of the membrane lipids and the temperature-dependence for alanine transport. The results are discussed in terms of membrane fatty acid composition, ordered in equilibrium fluid phase transitions and amino acid transport.

Paradoxical effects of protein synthesis inhibitors on uridine uptake in cultured cells: possible role of uncharged tRNA in regulating metabolism

Previous studies (J. Biol. Chem, 253: 99--105, 1978) showed that thyrotropin-releasing hormone (TRH) acutely stimulated uridine uptake in pituitary cell (GH 4Cl)) cultures. Studies on the role of protein synthesis in this response to TRH led to the finding that an inhibitor of ribosomal translation, cycloheximide, also stimulated uridine uptake acutely. Studies reported here attempt to determine the mechanism of cycloheximide action and whether cycloheximide and hormone stimulation of uridine uptake occurred by similar pathways. The experiments presented indicate that: (1) seven inhibitors of ribosomal translation stimulated uridine uptake; (2) in contrast, inhibition of protein synthesis at tRNA aminoacylation resulted in reduced rates of uridine uptake; (3) inhibition of tRNA aminoacylation blocked cycloheximide but not TRH stimulation of uptake; (4) cycloheximide stimulation of uptake was restricted to amino acid-depleted cultures; (5) amino acid supplementation stimulated uridine uptake with a time-course identical to that of cycloheximide; (6) cycloheximide and amino acid supplementation promoted reacylation of cellular tRNAs in amino acid-depleted cultures; and (7) cycloheximide stimulation of uridine uptake resulted from enhanced nucleoside phosphorylation rather than increased uridine transport. We conclude that cycloheximide and amino acid stimulation of uridine phosphorylation may be mediated through a common pathway involving the extent of amino-acylation of cellular tRNAs. Furthermore, cycloheximide and TRH stimulate uridine phosphorylation by pathways that are distinguishable. It is apparent that not all cellular effects of cycloheximide can be attributed solely to inhibition of the synthesis of proteins.