Serum-mediated regulation of amino acid transport in cultured chick embryo fibroblasts

Transforming growth factor-beta 1 stimulates vascular smooth muscle cell L-proline transport by inducing system A amino acid transporter 2 (SAT2) gene expression

Transforming growth factor-beta1 (TGF-beta 1) is a multifunctional cytokine that contributes to arterial remodelling by stimulating vascular smooth muscle cell (SMC) growth and collagen synthesis at sites of vascular injury. Since l-proline is essential for the synthesis of collagen, we examined whether TGF-beta 1 regulates the transcellular transport of l-proline by vascular SMCs. l-Proline uptake by vascular SMCs was primarily sodium-dependent, pH-sensitive, blocked by neutral amino acids and alpha-(methylamino)isobutyric acid, and exhibited trans-inhibition. Treatment of SMCs with TGF-beta 1 stimulated l-proline transport in a concentration- and time-dependent manner. The TGF-beta 1-mediated l-proline uptake was inhibited by cycloheximide or actinomycin D. Kinetic studies indicated that TGF-beta 1-induced l-proline transport was mediated by an increase in transport capacity independent of any changes in the affinity for l-proline. TGF-beta 1 stimulated the expression of system A amino acid transporter 2 (SAT2) mRNA in a time-dependent fashion that paralleled the increase in l-proline transport. Reverse transcriptase PCR failed to detect the presence of SAT1 or amino acid transporter 3 (ATA3) in either untreated or TGF-beta 1-treated SMCs. These results demonstrate that l-proline transport by vascular SMCs is mediated predominantly by the SAT and that TGF-beta 1 stimulates SMC l-proline uptake by inducing the expression of the SAT2 gene. The ability of TGF-beta 1 to induce SAT2 expression may function to provide SMCs with the necessary levels of l-proline required for collagen synthesis and cell growth.

Regulatory and molecular aspects of mammalian amino acid transport

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The relationship between sodium-dependent transport of anionic amino acids and cell proliferation

The relationship between the transport of anionic amino acids and the proliferative status of the cell population has been studied in NIH-3T3 cells. Proliferative quiescence, verified by determinations of growth-rate quotient and incorporation of thymidine, is associated with a marked increase of the influx of L-aspartate. After 7-10 days of serum starvation, the initial influx of L-aspartate increases by 8-10-times with respect to the transport activity determined in growing cells. The operational properties of the influx of L-aspartate are similar in growing and quiescent cells; in particular, the influx of the anionic amino acid is mostly Na(+)-dependent and completely suppressed by an excess of L-glutamate and D-aspartate, but not of D-glutamate. These features suggest that, in both cases, aspartate uptake occurs through system X(-)AG. The quiescence-related increase in aspartate transport is gradual, sensitive to the inhibition of protein synthesis and referable to the enhanced maximal capacity of transport system X(-)AG. Restoration of serum concentration in the culture medium of serum-starved cells causes a decrease in aspartate transport that is maximal in correspondence to late G1/S phases. It is concluded that the X(-)AG system for anionic amino-acid uptake is sensitive to the proliferative status of the cell population and that, in particular, its transport activity is stimulated by the establishment of proliferative quiescence.

Analysis of neutral amino acid transport systems in the small intestine: a study of brush border membrane vesicles

Transport of L-proline, L-leucine and L-cysteine was studied in brush border membrane vesicles prepared from guinea pig ileum. Concentrative transport of L-proline, L-leucine and L-cysteine was obtained in the presence of an Na+ gradient from, outside to inside of the vesicles, which indicated contribution of either system A (alanine-preferring) or system ASC (alanine-, serine- and cysteine-preferring) to the transport. When Na+ was replaced by Li+, L-leucine and L-cysteine maintained the same concentrative transport. However, the concentrative transport of L-proline was markedly decreased by Li+ -for-Na+ substitution. Strong exchange properties of L-leucine transport via system L (leucine-preferring) was observed with brush border membrane vesicles, in which preloaded L-methionine could be exchanged with labeled L-leucine added outside the vesicles. These results suggest that the small intestine of the guinea pig possesses classical neutral amino acid transport systems such as systems A, ASC and L.

Induction of amino acid transport activity in chick embryo fibroblasts by replacement of extracellular sodium chloride with disaccharide

The activity of amino acid transport System A in avian fibroblasts was increased following incubation of the cells in a medium in which most of the NaCl normally present had been isoosmotically replaced by sucrose. This increase was detectable after 2 h of incubation, reached a maximum at about 4 h, and remained constant thereafter. Transfer of treated cells back to a normal medium resulted in decay of the induced transport activity, with a half-life of less than 2 h. Kinetic analysis revealed that the increase in transport activity arose from an increase in Vmax, with little change in Km. This induction of System A activity did not occur if an inhibitor of either RNA or protein synthesis was present in the modified medium. The use of various different solutes as replacements for NaCl in the incubation medium showed that, although each replacement caused a decrease in both cellular Na+ content and protein synthesis, only disaccharides produced the increase in amino acid transport activity. In addition, estimates of cell volume indicated that, even under iso-osmotic conditions, incubation in the sucrose-containing medium caused initial cell shrinkage, followed by swelling. It is concluded that this induction of System A activity is associated with a volume regulatory process and that this process probably accounts for the parallel responses previously observed when cells were incubated in hyperosmolar media. Induction of amino acid transport activity by this process is distinct from adaptive regulation, caused by amino acid starvation; but the two processes are not strictly additive, and so appear to converge at some step.

Hyperosmolarity-induced stress proteins in chick embryo fibroblasts

The effects of a short exposure of chick embryo fibroblasts to a hyperosmolar medium on monovalent cation content, rate of protein synthesis, and polypeptide pattern expression were studied. The hyperosmolar shock gave an immediate and pronounced inhibition of the protein-synthesis rate temporally related to a marked alteration of the intracellular Na+ content. Following the return of the cells to an osmolar environment, the internal Na+ content quickly resumed its previous level, while the recovery of the protein-synthesis rate was more gradual. During the recovery period, there was enhanced expression of at least 12 proteins. The 4 major induced proteins exhibited apparent molecular weights of 96, 87, 70, and 48 kDa. A reduction in the synthesis of five protein bands including three large polypeptides of 220, 160, and 140 kDa was also observed. A comparison with the 3 major proteins induced by a 44 degrees C heat shock indicated an apparent similarity with only two of the hyperosmolarity-inducible polypeptides. Moreover, evidence has been also obtained of the close similarity between the 96 and 75 kDa glucose-regulated proteins and the 96 and 75 kDa proteins inducible by a hyperosmolar shock or by a continuous hyperosmolar treatment, respectively. The kinetics of the stress-proteins appearance indicated nonsimultaneous induction. The presence of actinomycin D during the exposure of the cells to the stress and the recovery period suggested that the expression of some hyperosmolarity-enhanced proteins is regulated at the transcriptional level.

Adaptive response of cultured fibroblasts to hyperosmolarity

Raising to 0.4 osM the osmolarity of the medium in which chick embryo fibroblasts are incubated quickly increases the internal Na+ concentration, inhibits protein synthesis and also stimulates amino acid transport. On extending the incubation time, cells appear to adapt to the altered environment, as the Na+ content declines toward control values within few hours. Protein synthesis resumes its normal rate within 12-14 h of treatment. Experimental alteration of the monovalent cation content by substituting extracellular Na+ with other osmolites or by using ouabain or the ionophore monensin reveals an impairment of protein synthesis. Analysis by SDS-PAGE reveals an alteration of the polypeptide pattern expressed by hyperosmolarity-exposed cells, resulting in an enhanced synthesis of the 87, 75 and 53 kD proteins and inhibition of a 125 kD band. The previously increased amino acid transport activity also reverts to its normal level, but only after 40-50 h of incubation. The growth rate of these cells does not appear to be significantly affected during the first 3 days of the hyperosmolar treatment. Results presented in this publication identify the alteration of the protein synthesis rate, the change in the intracellular cation content and the increase in amino acid transport activity as plausible parameters of adaptive response, and suggest that the modulation of gene expression observed in cells exposed continuously to hyperosmolarity may be a consequence of the alteration of the intracellular monovalent cation concentration.

Anthracycline resistance and consequences of the in situ-in vitro transfer

Adriamycin-resistant and normal cells of the sarcoma 180 of the mouse undergo qualitatively different deflections from the in situ state when prepared for an experiment. Resistant cells perform a fast reactive decline in the proliferative activity. They are capable of quiescence as defined by the time needed for the induction of the proliferation. Sensitive cells seem to be unable to quiesce and are only slowed down. These facts must be taken into account in interpretation of similar results. Differences in experiments need not necessarily imply differences in situ. Such in vitro appearing differences between sensitive and adriamycin-resistant cells of the murine sarcoma 180 include the retention of the mitochondria-specific stain rhodamine 123 and the uptake of anthracyclines, both being reduced in resistant cells. After labeling sensitive cells with thymidine in vivo and sorting them according to their rhodamine 123-derived fluorescence, the label was only found in the major, highly fluorescing fraction. A small low-fluorescing fraction remained unlabeled. We were able to demonstrate similar results with labeled anthracyclines applied to both the sensitive and the resistant cells in a short period between the removal of the cells from the ascites and the cell sorting. The adriamycin resistance seems to be joined with the ability of the cells to reduce their proliferative activity following changes to unfavorable conditions in vitro. Quiescent cells of the resistant line demonstrate the "anthracycline pump." Substances which are known to increase the sensitivity of anthracycline-resistant cells (TWEEN, verapamil) also shift the cells from low to high rhodamine 123-fluorescence.

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.

The effect of the intracellular sodium level on the activity of amino acid transport systems L and A in SV40 3T3 cells

The rate of transport of phenylalanine and leucine, pertinent amino acids of System L, has been measured in SV40 3T3 cells as a function of the presence of Na+ ions during the reloading phase that precedes the influx determination. The presence of Na+ ions during the reloading phase resulted in an increase of the subsequent substrate influx through System L. This effect was related to the intracellular Na+ level and was found to be independent by the presence of a chemical sodium gradient outside-inside during influx determination; furthermore, this effect could not be ascribed to a difference between control and Na+-treated cells in the internal levels of those amino acids that participate in the exchange phenomena of transport System L. The transport of phenylalanine appeared to have the ability to accept Li+ for Na+ substitution in the 'trans' position. The presence of Na+ ions in the 'trans' position was not required to optimize the transport of System A-reactive substrates, whose influxes are dependent on the presence of the cation in 'cis' position. Analysis of the relationship between influx and substrate concentration indicated that the Na+-dependent increase of substrate influx was associated with an enlarged capacity of the high-affinity component of transport System L.

Neutral amino acid transport in embryonal carcinoma cells

Neutral amino acid transport was characterized in the pluripotent embryonal carcinoma (EC) cell line, OC15. Ten of the thirteen amino acids tested are transported by all three of the major neutral amino acid transport systems--A, L, and ASC--although one system may make a barely measurable contribution in some cases. The characterization of N-methyl-aminoisobutyric acid (meAIB) transport points to this model amino acid as a definitive substrate for System A transport by OC15 cells. Thus, high concentrations of meAIB can be used selectively to block System A transport, and the transport characteristics of meAIB represent system A transport. Kinetic analysis of System A, with a Km = 0.79mM and Vmax = 14.4 nmol/mg protein/5 min, suggests a single-component transport system, which is sensitive to pH changes. While proline transport in most mammalian cells is largely accomplished through System A, it is about equally divided between Systems A and ASC in OC15 cells, and System A does not contribute at all to proline transport by F9 cells, an EC cell line with limited developmental potential. Kinetic analysis of System L transport, represented by Na+-independent leucine transport, reveals a high-affinity, single-component system. This transport system is relatively insensitive to pH changes and has a Km = 0.0031 mM and Vmax = 0.213 nmol/mg protein/min. The putative System L substrate, 2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH), inhibits Systems A and ASC as well as System L in OC15 cells. Therefore, BCH cannot be used as a definitive substrate for System L in OC15 cells. Phenylalanine is primarily transported by Na+-dependent Systems A and ASC (83% Na+-dependent; 73% System ASC) in OC15 cells, while it is transported primarily by the Na+-independent System L in most other cell types, including early cleavage stage mouse embryos and F9 cells. We have also found this unusually strong Na+-dependency of phenylalanine transport in mouse uterine blastocysts (82% Na+-dependent). There is no evidence for System N transport by OC15 cells, since histidine is transported primarily by a Na+-independent, BCH-inhibitable mechanism.

Adaptive regulatory control of System A transport activity in a kidney epithelial cell line (MDCK) and in a transformed variant (MDCK-T1)

Adaptive regulatory control of System A activity was investigated using MDCK cells and a chemically induced, oncogenic transformant of MDCK cells, MDCK-T1. Within 7 hours after transfer to an amino-acid-deficient medium, A activity of subconfluent MDCK cells had maximally derepressed, but this activity in confluent MDCK cells and in subconfluent transformed cells showed little capacity for derepression. Amino-acid-starved, subconfluent MDCK cells were used to study trans-inhibition and repression of A activity by individual amino acids. Trans-inhibition and repression were defined as the cycloheximide-insensitive and cycloheximide-sensitive components, respectively, of the total inhibition. Trans-inhibition correlated well with substrate affinity, but repression did not. Trans-inhibition and repression were further characterized using alpha-(methylamino) isobutyric acid (mAIB), a trans-inhibitor, and glutamate, an effective repressor. The apparent initial T 1/2 for inhibition by mAIB in the presence of cycloheximide was 0.5 hours, while that for repression by glutamate was 4.7 hours. Half-maximal inhibition by mAIB and repression by glutamate occurred at approximately 0.02 mM and 0.07 mM, respectively. Reversal of trans-inhibition by methionine occurred in the presence of cycloheximide within 1-4 hours after removal of methionine. The A system of the transformed MDCK-T1 cells showed elevated activity, little capacity for derepression, resistance to repression by amino acids, but retention of sensitivity to trans-inhibition. Kinetic analysis of mAIB uptake indicated that the A system of MDCK-T1 cells has become kinetically more complex in a manner which resembled amino-acid-starved rather than amino-acid-fed MDCK cells. These results suggest that the A system of MDCK-T1 cells has become resistant to adaptive regulatory control.

Hormonal regulation of the System A amino acid transport adaptive response mechanism in a kidney epithelial cell line (MDCK)

When mammalian cells are starved for amino acids, the activity of the A amino acid transport system increases, a phenomenon called adaptive regulation. We have examined the effects of those factors which support Madin-Darby canine kidney (MDCK) cell growth in a defined medium on the derepression of System A activity. Of the five factors which supported MDCK cell growth, insulin was found to be an absolute requirement for derepression. In contrast, PGE1 was a negative controlling factor for the transport system. Growth of MDCK cells in the absence of PGE1 resulted in elevated System A activity which derepressed poorly upon amino acid starvation. Kinetic analysis of alpha-(methylamino) isobutyric acid (mAIB) uptake as a function of substrate concentration showed that the elevated A activity observed when cells were grown in the absence of PGE1 was kinetically similar to the activity induced by starvation for amino acids. Transport of mAIB by amino-acid-fed cells grown in the presence of PGE1 was characterized by a linear Eadie-Hofstee graph and by a relatively low Vmax. Transport by cells starved for amino acids or by cells grown in the absence of PGE1 was characterized by biphasic kinetics for mAIB transport and by elevated Vmax values. An influence of growth factors on the inactivation of derepressed A activity was also observed. In the presence of cycloheximide the rate of loss of A activity in amino-acid-starved cells was 1/4-1/2 that of amino-acid-fed cells. Insulin slowed inactivation in the absence of most amino acids in a protein-synthesis-independent manner, but insulin did not influence the more rapid inactivation observed in amino-acid-fed cells. These results indicate that the level of System A activity observed in response to regulation by amino acids represents a balance between carrier synthesis and inactivation, which can be positively or negatively influenced by growth factors.

Effect of hyperosmolarity on the activity of amino acid transport system L in avian fibroblasts

The transport of selected neutral amino acids known as good substrates of amino acid transport System L has been studied in chick embryo fibroblasts exposed for 4 hours to hyperosmolar culture medium. The activity of the L system, as measured by initial rates of L-phenylalanine uptake, increased in hyperosmolarity treated cells when determined before any cell depletion of intracellular amino acids. This effect was lost after depletion but reappeared after reloading the cells with pertinent substrates of System L. This transport activity appeared to be related to the internal level of amino acids capable of exchange through System L. In hyperosmolarity-treated chick embryo fibroblasts a higher level of System L substrates was obtained during the reloading phase in comparison to control cells. This expanded amino acid pool reflected an increased activity of transport System A, an agency of amino acid mediation known to enlarge its capacity following a hyperosmolar treatment of chick embryo fibroblasts (see Tramacere et al., 1984). L-Methionine, a preferred substrate of both A and L systems, appeared to be involved in the coupling between the activity of amino acid transport Systems A and L in these cells.

Osmoregulation of amino acid transport activity in cultured fibroblasts

The effect of exposure of chick embryo cells to increasing concentrations of Na+ in the culture medium on the subsequent amino acid transport as determined at physiological osmolarity was investigated in detail. It was found that the hyperosmolar treatment stimulated amino acid transport in a dose-dependent manner up to 200 mM Na+. Changes were measurable as early as 1 h after altering Na+ and reached a maximum after 4 h, remaining constant thereafter. The maintenance of this effect required continuous exposure of the cell to high Na+ in the culture medium. Hyperosmolarity-mediated increases in amino acid transport activity by system A have been detected with L-proline and L-alanine. Transport activities of systems ASC and L did not change appreciably after exposure of the cells to high Na+. Inhibition of protein synthesis by cycloheximide or RNA synthesis by actinomycin D (actD) prevented these uptake changes. Kinetic analysis indicated that the stimulation of the activity of transport system A by high Na+ treatment occurred through a mechanism affecting Vmax rather than Km.

Cell membrane amino acid transport processes in the domestic fowl (Gallus domesticus)

Intestinal absorption of amino acids in the chicken occurs by way of processes which are concentrative, Na+-dependent and dependent upon metabolic energy in the form of ATP. Intestinal transport is carrier-mediated, subject to exchange transport (trans-membrane effects) and is inhibitable by sugars, reagents which inactivate sulfhydryl groups, potassium ion, and by deoxpyridoxine, an anti-vitamin B6 agent. It is stimulated by phlorizin, a potent inhibitor of sugar transport, and in Na+-leached tissue by modifiers of tissue cyclic AMP levels, e.g. theophylline, histamine, carbachol and secretin. Separate transport sites with broad, overlapping specificities function in the intestinal absorption of the various classes of common amino acids. A simple model for these sites includes one for leucine and other neutral amino acids, one for proline, beta-alanine and related imino and amino acids, one for basic amino acids, and one for acidic amino acids. Absorption of amino acids appears to be widespread in occurrence in the digestive tract of the domestic fowl; transport has been reported to be present in the crop, gizzard, proventriculus, small intestine and in the colon. By the end of the first week of life post-hatch, the caecum loses its ability to transport. Similarly, the yolk sac loses its ability by the second day post-hatch. Intestinal transport was noted before hatch and was found to be maximal immediately post-hatch. A requirement for Ca2+ appears to be lost after the first week of life post-hatch. The cationic amino acids appear to be reabsorbed by a common mechanism in the kidney. Transport rates of leucine measured in the intestine or in the erythrocyte were found to cluster about discrete values when many individual chickens were surveyed; such patterns may be an expression of gene differences between individuals. Two lines of chickens have been developed, one high and the other low uptake, through selective breeding based on the ability of individual birds to absorb leucine in erythrocytes. High leucine absorbing chickens were found to be more effective in absorbing lysine and glycine, were more effectively stimulated by Na+, had greater erythrocyte Na+, K+-ATPase activity, and their erythrocytes contained about 20% less Na+ than low line erythrocytes. The underlying genetic difference between these lines may reside at the level of the Na+, K+-ATPase and (or) with a regulatory gene determining carrier copies. Amino acid transport in erythrocytes was noted to be highest in pre-hatch chicks and to diminish during post-hatch development.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Membrane transport during erythroid differentiation

Transport, unidirectional flux, of a monosaccharide, a nucleoside and three amino acids, all of which enter cells by independent, discrete carriers, was compared at three stages of erythroid maturation, the normal (anucleate) mouse erythrocyte, and in differentiated and undifferentiated Friend erythroleukemia cells. We found specific transport alterations during this developmental program. Transport of 3-O-methylglucose increased with each successive developmental stage. Aminoisobutyrate transport was maintained during Friend cell differentiation, but fell slightly in erythrocytes. Leucine, lysine and uridine transport began to fall two days after dimethylsulfoxide exposure, and diminished further in red cells. These studies of transport are not directly comparable to uptake studies reported by others. Median cell volume and thus surface area decreased more during differentiation than amino acid transport declined, so flux, transport past a unit area of membrane, actually increased. Monosaccharide flux also increased. Only uridine transport fell in parallel to surface area. Perhaps sites for nutrient transport required for energy production are preferentially maintained.

Schistosoma mansoni: in vitro uptake and incorporation of glycine by adult worms

1. The uptake of [3H]glycine into an ethanol-extractable intracellular pool and its simultaneous incorporation into protein (TCA insoluble fraction) have been measured in adult male and female Schistosoma mansoni incubated, in vitro, in complex media. 2. The patterns of uptake and incorporation were similar in male and female schistosomes incubated in media containing serum, and also in male worms incubated in the presence or absence of serum. 3. The amounts of [3H]glycine uptake and incorporation were directly proportional to the extracellular glycine concentration and were identical in male worms (in the presence or absence of serum) and female worms. 4. Emetine at a concentration producing a 95% inhibition of protein synthesis was without effect on the uptake of [3H]glycine into the ethanol-extractable pool in male worms.

Serum stimulation of plasma protein synthesis in culture is selective and rapidly reversible

Primary hepatocyte monolayers, derived from chick embryos, can be cultured from the onset in a completely chemically defined medium, free of added hormones. The liver cells synthesize and secrete a wide spectrum of plasma proteins for several days in this serum-free environment. Addition of fetal bovine serum elicits a 3-5-fold increase in the production of certain plasma proteins: fibrinogen, albumin, and the alpha1-globulin M. This effect of serum is selective; transferrin and plasminogen syntheses are enhanced less than 1.5-fold. Significant stimulation is observed with 0.1% fetal bovine serum, and half-maximal values for individual plasma proteins are obtained with concentrations ranging between 0.4 and 1%. The stimulatory activity of serum shows no developmental or species specificity. Plasma is active as serum derived from the same blood sample. The hepatocytes respond rapidly to serum, significant changes in albumin synthesis occurring less than 1 h after serum addition or removal. The effect of short exposure is fully reversible. These results establish the capacity of low concentrations of serum to stimulate plasma protein synthesis and underscore the importance of studying the effects of hormones and other factors under serum-free conditions. The findings suggest that, in addition to the classical hormones, ubiquitous but as yet uncharacterized serum components play a role in controlling this major hepatic function.

Amino acid transport and rubidium-ion uptake in monolayer cultures of hepatocytes from neonatal rats

Amino acid and K(+) transport during development has been investigated in hepatocyte monolayer cultures with either alpha-amino[1-(14)C]isobutyrate or (86)Rb(+) used as a tracer for K(+). Parenchymal cells from neo- and post-natal rat livers have been isolated by an improved non-perfusion technique [Bellemann, Gebhardt & Mecke (1977)Anal.Biochem.81, 408-415], and the resulting hepatocyte suspensions purified from non-hepatocytes before inoculation. In the presence of Na(+) (Na(+)-dependent component), the rates of amino acid uptake in neonatal hepatocytes were markedly enhanced compared with cells from 30-day-old rats. When Na(+) was replaced by choline (Na(+)-independent component) the accumulation of alpha-aminoisobutyrate was decreased and it was not affected by the age of the animals. Kinetic analysis of Na(+)-dependent alpha-aminoisobutyrate transport revealed the existence of a high-affinity low-K(m) component (K(m)0.91mm) with a V(max.) of 2.44nmol/mg of protein per 4min, which later declined gradually with progressive development. Rates of Rb(+) transport were concomitantly enhanced in neonatal hepatocytes and thereafter declined with postnatal age. The increased Rb(+) influx was effectively inhibited by ouabain and reflected elevated activity of the electrogenic Na(+)/K(+)-pump during early stages of development. Kinetic evaluation of the enhanced rates of Rb(+) uptake indicates multiple and co-operative binding sites of the enzyme involved in the Rb(+) uptake, and the transport system is positively co-operative (the Hill coefficient h is >1.0). In short, amino acid transport in neonatal rat hepatocytes is increased as a result of an existing low-K(m) component for the Na(+)-dependent alpha-aminoisobutyrate uptake, which endows the hepatocytes with a high capability for concentrating amino acids at low ambient values. The concomitant enhancement of K(+) transport reflects changes in the electrochemical gradient for Na(+) across the hepatocellular membrane and, along with this, presumably alterations in the membrane potential; the latter might be the driving force for the enhanced alpha-aminoisobutyrate transport in the alanine-preferring system during postnatal age.

Active Ca2+ transport by membrane vesicles from pigeon erythrocytes. Stimulation by amino acids, ATP, GTP, Pi and some other cell constituents

Pretreatment of pigeon erythrocyte membrane vesicles with amino acids, ATP, GTP, Pi and some other simple cell constituents (singly and in combination) causes an increase in ATP-dependent Ca2+-uptake activity of vesicles upon subsequent incubation with 45Ca2+ after removal of the above agents from the 'i' face. Amino acids augment the stimulation by all stimulatory agents and are required for stimulation by Pi. The effects of amino acids, ATP, GTP and Pi all occur at physiological concentrations. Many if not all of the effects of the mixture of amino acids that occur naturally in the cells can be accounted for by the group transported by the 'ASC' transport system of Christensen (Christensen, H.N. (1975) Biological Transport, 2nd edn., W.A. Benjamin, Inc., Reading, MA), but not by any single amino acid at its physiological concentration. The effects of ATP and GTP are not mimicked by their non-hydrolysable beta,gamma-imido analogues not by the corresponding 3',5'-cyclic monophosphates. None of the effects described appears to involve calmodulin. We suggest that amino acid transport plays a role in metabolic regulation through effects on cell [Ca2+]. Analogous effects on cell [Ca2+] may be involved in the action of the many hormones which augment amino acid accumulation by the 'A' amino acid transport system.

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.

Cell density and amino acid transport in 3T3, SV3T3, and SV3T3 revertant cells

The transport of selected neutral and cationic amino acids has been studied in Balb/c 3T3, SV3T3, and SV3T3 revertant cell lines. After properly timed preincubations to control the size of internal amino acid pools, the activity of systems A, ASC, L, and Ly+ has been discriminated by measurements of amino acid uptake (initial entry rate) in the presence and absence of sodium and of transport-specific model substrates. L-Proline, 2-aminoisobutyric acid, and glycine were primarily taken up by system A; L-alanine and L-serine by system ASC; L-phenylalanine by system L; and L-lysine by system Ly+ in SV3T3 cells. L-Proline and L-serine were also preferential substrates of systems A and ASC, respectively, in 3T3 and SV3T3 revertant cells. Transport activity of the Na+-dependent systems A and ASC decreased markedly with the increase of cell density, whereas the activity of the Na+-independent systems L and Ly+ remained substantially unchanged. The density-dependent change in activity of system A occurred through a mechanism affecting transport maximum (Vmax) rather than substrate concentration for half-maximal velocity (Km). Transport activity of systems A and ASC was several-fold higher in transformed SV3T3 cells than in 3T3 parental cells at all the culture densities that could be compared. In SV3T3 revertant cells, transport activity by these systems remained substantially similar to that observed in transformed SV3T3 cells. The results presented here add cell density as a regulatory factor of the activity of systems A and ASC, and show that this control mechanism of amino acid transport is maintained in SV40 virus-transformed 3T3 cells that have lost density-dependent inhibition of growth, as well as in SV3T3 revertant cells that have resumed it.

Regulation of amino acid transport in chicken embryo fibroblasts by purified multiplication-stimulating activity (MSA)

Enhanced amino acid transport is observed when quiescent cultures of chicken embryo fibroblasts are stimulated to proliferate by the addition of purified multiplication-stimulating activity (MSA). This increase in amino acid transport is an early event occurring prior to the onset of DNA synthesis in stimulated cells. Results indicate that the changes in transport activity, as measured by alpha-aminoisobutyric acid (AIB) uptake, are due to stimulation of only the Na+-dependent A transport system. There is little or no change in the activities of transport systems ASC, L, or Ly+ upon exposure to MSA. A kinetic analysis shows this increased activity is due to a change in Vmax while Km remains unaltered. Continuous exposure to the stimulus is required to maintain the increased level of transport activity and the presence of inhibitors of RNA and protein synthesis significantly inhibits the response. Results also indicate that a similar specific increase in the A transport system is initiated when RSV tsNY68 infected cells are shifted to the permissive temperature. It appears that the A system of mediation is emerging as a strategic regulatory site for cell function.

Diabetes and cancer: a postulated relationship

Diabetes leads to periodic fluctuations in levels of nutrients and cyclic nucleotides in plasma and various tissues. Variations in cyclic nucleotide levels, periods of nutrient elevation and surges of insulin combine to stimulate growth. Growth stimulation may be associated with the promotional phase of carcinogenesis. Diabetes should therefore predispose the individual to carcinogenesis and limited epidemiological data available suggest that it does.

Oxocarboxylic acids, pyridine nucleotide-linked oxidoreductases and serum factors in regulation of cell proliferation

When serum is made rate-limiting for clonal multiplication of human diploid fibroblasts, the presence of a 2-oxocarboxylic acid in the medium becomes essential. The requirement is independent of the 20 amino acids and glucose. Glyoxylic, pyruvic, 2-oxoglutaric, and oxalacetic acids are most effective. The types of 2-oxocarboxylic acids that support multiplication are oxidized substrates for several, pyridine nucleotide-linked intracellular oxidoreductases. The requirement is not satisfied by carboxylic acids, oxidized substrates for oxidoreductases that are not lniked to pyridine nucleotides, or by nonspecific electron acceptors. The quantitative requirement for 2-oxocarboxylic acids in cell multiplication is markedly affected by the concentration of serum proteins in the medium. Therefore, 2-oxocarboxylic acid metabolism may be related to the mechanism by which serum growth factors regulate cell multiplication.

Serum-dependent regulation of alpha-aminoisobutyric acid uptake in bovine granulosa cells

The removal of serum from the medium of ovarian granulosa cells in exponential or confluent stages of growth results in a rapid and pronounced decrease in the rate of transport of the non-metabolizable amino acid, alpha-aminoisobutyric acid. This decrease is rapidly and completely reversed by the addition of serum. The decrease and its reversal are insensitive to inhibitors of RNA and protein syntheisis and are unaffected by a number of other metabolic inhibitors. The serum requirement cannot be replaced by peptide hormones known to stimulate cell division and secretion by these cells. These data are consistent with a model of post-translational control of AIB transport by a high-molecular-weight component of serum.

The growth of heart cells in culture. Evidences for a multiple activation of the pleiotypic program

Each medium renewal of confluent primary heart cell cultures derived from new born rats induces a pleiotypic response which leads to active proliferation. The presence of serum in the culture medium is essential for this activation of growth. Nutrient starvation prior to the activation decreases the response of the cells to serum. Serum starvation prior to the activation increases the serum dependence of the incorporation of labelled leucine but leaves the serum dependence of DNA synthesis unchanged. Ageing in culture decreases the serum dependence of the incorporation of labelled thymidine and amino acids but maintains it for alpha amino isobutyric acid transport. Several active components in human serum were distinguished by fractionated dialysis. A single dialyzable component stimulates both thymidine and amino acid incorporations. The transport of 2 deoxy-D-glucose is activated by another rapidly dialyzing component. The activation of alpha amino isobutyric acid transport may result from several components that are distanct from the previous ones. These results imply that a multiplicity of controls underly the pleiotypic activation of heart cell cultures by medium changes.