Repression, derepression, transinhibition, and trans-stimulation of amino acid transport in rat hepatocytes and four rat hepatoma cell lines in culture

Single liposome analysis of peptide translocation by the ABC transporter TAPL

ATP-binding cassette (ABC) transporters use ATP to drive solute transport across biological membranes. Members of this superfamily have crucial roles in cell physiology, and some of the transporters are linked to severe diseases. However, understanding of the transport mechanism, especially of human ABC exporters, is scarce. We reconstituted the human lysosomal polypeptide ABC transporter TAPL, expressed in Pichia pastoris, into lipid vesicles (liposomes) and performed explicit transport measurements. We analyzed solute transport at the single liposome level by monitoring the coincident fluorescence of solutes and proteoliposomes in the focal volume of a confocal microscope. We determined a turnover number of eight peptides per minute, which is two orders of magnitude higher than previously estimated from macroscopic measurements. Moreover, we show that TAPL translocates peptides against a large concentration gradient. Maximal filling is not limited by an electrochemical gradient but by trans-inhibition. Countertransport and reversibility studies demonstrate that peptide translocation is a strictly unidirectional process. Altogether, these data are included in a refined model of solute transport by ABC exporters.

Structural basis of trans-inhibition in a molybdate/tungstate ABC transporter

Transport across cellular membranes is an essential process that is catalyzed by diverse membrane transport proteins. The turnover rates of certain transporters are inhibited by their substrates in a process termed trans-inhibition, whose structural basis is poorly understood. We present the crystal structure of a molybdate/tungstate ABC transporter (ModBC) from Methanosarcina acetivorans in a trans-inhibited state. The regulatory domains of the nucleotide-binding subunits are in close contact and provide two oxyanion binding pockets at the shared interface. By specifically binding to these pockets, molybdate or tungstate prevent adenosine triphosphatase activity and lock the transporter in an inward-facing conformation, with the catalytic motifs of the nucleotide-binding domains separated. This allosteric effect prevents the transporter from switching between the inward-facing and the outward-facing states, thus interfering with the alternating access and release mechanism.

Adaptive regulation of amino acid transport in nutrient-deprived human hepatomas

BACKGROUND

Malignant cells require increased amounts of amino acids, in particular glutamine and leucine, to support DNA and protein biosynthesis. Although plasma concentrations in the center of solid tumors can be much lower than normal circulating levels, it is still unknown how tumor cells can survive despite low amino acid levels. We examined the effects of glutamine or leucine deprivation on cell growth and amino acid transport activity in two human hepatoma cell lines, SK-Hep and HepG2.

METHODS

We studied the transport of glutamine, leucine, alanine, and arginine. The carrier-mediated uptake of 3H-amino acids was determined in cells cultured in normal and amino acid-deprived media.

RESULTS

The growth of both cell lines was dependent on the concentration of glutamine and leucine. In SK-Hep, there was a significant increase in initial rate glutamine transport activity in the glutamine-deprived group, attributable to an increase in transporter affinity (Km; 0.6 mmol/L [control], 385 +/- 43 mumol/L; 0.1 mmol/L, 221 +/- 11 mumol/L; P < 0.01). At low glutamine concentration, the saturable Na(+)-independent uptake of leucine and arginine as well as the Na(+)-dependent uptake of alanine increased significantly in both SK-Hep and HepG2. Similarly, in leucine-deprived SK-Hep cells, leucine uptake increased twofold, but the change was attributable to an enhanced transporter capacity (Vmax; 0.2 mmol/L [control], 38,900 +/- 700; 0.0 mmol/L, 75,900 +/- 4,900 pmol/mg protein per minute; P < 0.001).

CONCLUSIONS

Adaptive increases in initial rate amino acid transport activities were elicited by glutamine and leucine deprivation in these two human hepatoma cell lines. Decreased extracellular amino acid levels encountered by tumors in vivo may elicit similar adaptive responses that contribute to the maintenance of cytoplasmic levels of amino acids essential for growth.

No evidence for a tumor necrosis factor alpha stimulated 2-methylaminoisobutyric acid uptake in hepatocyte monolayer

This study investigates the short-term effects of glucagon and human recombinant tumor necrosis factor alpha (TNF alpha) singly and in association on 2-methylaminoisobutyric acid (MeAIB) transport in hepatocyte monolayers. As expected, glucagon induced a time-dependent stimulation of MeAIB transport. In our experimental conditions, TNF alpha did not induce cytolysis. A 2 hour exposure to TNF alpha (0.05-500 ng/l) with or without glucagon (10(-9) to 10(-6) M) did not modify the basal or glucagon-stimulated MeAIB transport. Varying the duration of exposure to TNF alpha 5 ng/l up to 6 h was equally ineffective. The presence of hydrocortisone potentiated the glucagon-stimulated transport, but TNF alpha remained ineffective. Finally, the association of interferon (IFN gamma) with TNF alpha and/or glucagon was unable to modify the transport activity. These data demonstrate that TNF alpha does not exert a direct effect on MeAIB transport in hepatocytes, at least on a short-term basis.

Effects of glutamine deprivation on glutamine transport and synthesis in primary culture of rat skeletal muscle

The effects of deprivation and supplementation of exogenous glutamine (0.06 and 2.2 mM in the culture medium, respectively) were studied in mononucleated myoblasts and in multinucleated myotubes. Myoblasts cultured in glutamine-deprived medium showed reductions in plating efficiency and myotube fusion index. Myotubes grown in glutamine-supplemented cultures had higher intracellular glutamine concentrations than those grown in glutamine-deprived medium (67 +/- 4.2 vs. 46 +/- 3.6 nmol/mg cell protein, respectively) and glutamine-supplemented myotubes utilized glutamine, whereas glutamine-deprived myotubes released it. Glutamine deprivation for 12 h caused a significant, cycloheximide-blockable increase in the capacity for glutamine uptake via system Nm in both myoblasts and myotubes (maximum velocity increases of 23 +/- 5.3 and 35 +/- 4.2%, respectively), which was reversed by glutamine replenishment. Depriving myotubes of glutamine did not alter the kinetics of uptake of amino acid transport systems A, ASC, or L. Glutamine deprivation resulted in a threefold increase in glutamine synthetase activity, whereas glutaminase activity remained unchanged. System Nm and glutamine synthetase appear to undergo adaptive upregulation in glutamine-deprived muscle cells to compensate for the reduced exogenous glutamine supply.

Adaptive regulation of ascorbate transport in osteoblastic cells

Osteoblasts possess a concentrative L-ascorbate (vitamin C) uptake mechanism involving a Na(+)-dependent ascorbate transporter located in the plasma membrane. The transporter is specific for ascorbate and stereoselective for L-ascorbate over D-isoascorbate. The present study examined the effects of ascorbate supplementation and deprivation on the activity of this transport system. L-ascorbate transport activity was determined by measuring uptake of the vitamin by ROS 17/2.8 osteosarcoma cells during 1 minute incubations with 5 microM L-[14C]ascorbate. The initial rate of L-[14C]ascorbate uptake by ROS 17/2.8 cells grown for 18 h in L-ascorbate-replete medium was 89 +/- 8 nmol/g protein per minute. Following removal of L-ascorbate from the growth medium, the initial rate of uptake increased within 6 h to 126 +/- 13 nmol/g protein per minute. Conversely, the initial rate of uptake by cells grown in ascorbate-free medium decreased following the addition of L-ascorbate, but not D-isoascorbate, to the medium. The effect of ascorbate pretreatment was specific for ascorbate transport in that preincubation of cultures with L-ascorbate did not affect uptake of 2-deoxy-D-glucose. Kinetic analysis revealed that modulation of ascorbate transport arose from changes in the apparent maximum rate of transport (Vmax) without changes in the affinity of the transport system for L-ascorbate. These experiments are the first to show that ascorbate transport by osteoblastic cells responds to vitamin C deprivation and supplementation. Adaptation of transport activity to substrate availability may play an important role in the physiological regulation of intracellular ascorbate levels.

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.

Substrate-dependent adaptive regulation and trans-inhibition of System A-mediated amino acid transport. Studies using rat hepatoma plasma membrane vesicles

Substrate-dependent regulation of amino acid transport by System A occurs by both direct action at the carrier (trans-inhibition) and transcriptional control (adaptive regulation). While experiments with intact cells have led to working models that describe these regulatory phenomena, the use of subcellular approaches will serve to refine the present hypotheses. Adaptive induction of System A transport following amino acid starvation of cells was shown to be dependent on de novo RNA and protein synthesis, and the stimulated activity was shown to be retained in isolated plasma membrane vesicles. This stimulated transport activity was tightly associated with the plasma membrane, but could be solubilized by 4 M urea and 2.5% cholate, and recovered following reconstitution of the protein into artificial proteoliposomes. These data support the working hypothesis that adaptive induction of transport is the result of de novo synthesis and insertion into the plasma membrane of System A carrier protein. In contrast, the activity of System ASC in the vesicles from the amino acid starved cells was actually reduced by 2-5-fold when compared to amino acid-fed cells. A more rapid form of regulation of System A activity is trans-inhibition. The use of isolated plasma membrane vesicles demonstrated that trans-inhibition in whole cells did not survive membrane isolation. However, substrate loading of isolated membrane vesicles containing high levels of System A activity, produced trans-inhibition in a very specific manner in that System A substrates resulted in decreased transport activity, while those amino acids which are poor substrates for the System A carrier did not. Thus, trans-inhibition is not the result of a recycling process involving an intracellular pool of carriers, but rather can be accounted for by differences in the kinetics for amino acid binding and/or translocation on the two sides of the membrane.

Substrate regulation of ascorbate transport activity in astrocytes

Astrocytes possess a concentrative L-ascorbate (vitamin C) uptake mechanism involving a Na(+)-dependent L-ascorbate transporter located in the plasma membrane. The present experiments examined the effects of deprivation and supplementation of extracellular L-ascorbate on the activity of this transport system. Initial rates of L-ascorbate uptake were measured by incubating primary cultures of rat astrocytes with L-[14C]ascorbate for 1 min at 37 degrees C. We observed that the apparent maximal rate of uptake (Vmax) increased rapidly (less than 1 h) when cultured cells were deprived of L-ascorbate. In contrast, there was no change in the apparent affinity of the transport system for L-[14C]ascorbate. The increase in Vmax was reversed by addition of L-ascorbate, but not D-isoascorbate, to the medium. The effects of external ascorbate on ascorbate transport activity were specific in that preincubation of cultures with L-ascorbate did not affect uptake of 2-deoxy-D-[3H(G)]glucose. We conclude that the astroglial ascorbate transport system is modulated by changes in substrate availability. Regulation of transport activity may play a role in intracellular ascorbate homeostasis by compensating for regional differences and temporal fluctuations in external ascorbate levels.

Neutral amino acid transport in human synovial cells: substrate specificity of adaptative regulation and transinhibition

Neutral amino acid transport was characterized in human synovial cells. The amino acids tested are transported by all three major neutral amino acid transport systems, that is, A, L, and ASC. The model amino acid 2-aminoisobutyric acid (AIB) was found to be a strong specific substrate for system A in synovial cells. When cells were starved of amino acids, the activity of AIB transport increased, reaching a maximum within 1 h. The stimulation of transport activity was not blocked by cycloheximide and would thus appear to be related to a release from transinhibition. Similarly, the decrease in the activity of AIB transport observed after the addition of alpha-methyl-aminoisobutyric acid (meAIB) appeared to be related to transinhibition. However, using a different approach, that is, amino acid starvation followed by incubation with 10 mM meAIB and transfer to an amino acid-free medium with or without cycloheximide supplementation, a clear increase in AIB uptake, due both to derepression and a release from transinhibition, was observed. Unlike human fibroblasts, the depression of system A in these synovial cells was not serum-dependent. The process of derepression was observed only after preloading with meAIB. Neither AIB nor alanine produced this phenomenon. Moreover, alanine preloading led to a large increase in AIB transport activity due to a release from transinhibition. These observations indicate that the process of derepression and release from transinhibition are specific to the substrates present in the culture medium prior to amino acid starvation.

Adaptation to Pi deprivation of cell Na-dependent Pi uptake: a widespread process

Phosphate enters kidney proximal tubular cells through an apical sodium-phosphate cotransport; this activity (Vmax) increases during phosphate deprivation (Kidney Int. 18: 36-47, 1980). This study investigated the mechanism of phosphate uptake and its adaptation to phosphate deprivation in cultured cells from different origins (kidney, LLC-PK1 and MDCK cells; liver, Fao cells; heart, myocyte primary cultures). All cells exhibited a sodium-dependent phosphate uptake that was reduced (greater than 75%) by external sodium substitution and inhibited by ouabain (35%) and 2,4-dinitrophenol or KCN (80%). Phosphate deprivation (exposure to phosphate-free medium) increased sodium-dependent phosphate uptake by 1.8- to 5.8-fold and decreased cell inorganic phosphate and ATP contents (70-80 and 17-30%, respectively). The stimulation of phosphate uptake resulted from an increase in Vmax without change in Km and was dependent on gene transcription and protein synthesis because it was inhibited by cycloheximide and 3-deoxyadenosine. Thus a deprivation-stimulated, sodium-dependent phosphate transport was demonstrated in cells originating from distal kidney tubules, liver, and heart. The findings suggest that in hypophosphatemic diseases, impairment of renal proximal phosphate reabsorption might be only one expression of a widespread alteration of cell phosphate regulation.

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.

Regulation of amino acid transport in isolated rat hepatocytes during development

The effect of amino acid depletion or supplementation and the effect of glucagon and insulin on the amino acid transport mediated by system A were investigated by determining the uptake of either 2-amino [1-14C]isobutyric acid (AIB) or N-methyl 2-amino [1-14C]isobutyric acid (MeAIB) in rat hepatocytes, freshly isolated at different stages of pre- and postnatal development. The data obtained show that the Na+-dependent uptake was higher at the earliest developmental stages, and steadily decreased until the adult level. The hormones increased AlB and MeAIB uptake enhancing the Vmax, while the Km was unchanged. This effect was evident in cells from adult and 18-20-day-old fetuses, while no response was present before the 18th day of fetal life and in the perinatal period. Actinomycin D or cycloheximide abolished this hormone-dependent increase. A decrease in AlB and MeAIB transport after incubation in an amino acid-rich medium was demonstrated at all ages tested, but was particularly evident in the prenatal life. The increase in the activity of the system following amino acid starvation was shown to be mostly dependent from de novo protein synthesis in the fetal life; on the contrary in the adult the increase appeared to be more linked to the release from transinhibition of the transport.

Effect of nutritional status on the fatty acid composition of rat liver and cultured hepatocytes

The lipid concentration and fatty acid composition of the whole liver and of cultured hepatocytes isolated from the livers of rats fed ad libitum (fed), fasted for 24 hr (fasted), or fasted for 48 hr and then refed a fat-free, high carbohydrate diet for 48 hr (refed) was studied. Hepatocytes were maintained as monolayer cultures in serum-free, lipid-free media and their fatty acid composition was analyzed at 3, 24, 48, 72 and 96 hr. The livers of fed animals, as well as their hepatocytes, contained less total lipid than those from animals on either of the other dietary regimes. Livers of fasted animals had three times the amount of lipid found in the livers of fed animals, and the livers of refed animals contained five times the amount of lipid as the livers of fed animals (all based on mg lipid/g wet weight of liver). The fatty acid composition of hepatocytes after 3 hr of culturing was very similar to that of fresh liver when compared in each of the dietary regimes. However, while the fatty acid compositions of livers and hepatocytes from fed and fasted animals were similar, the pattern in liver of refed animals was quite distinct from that of the fed animals. In the fed and fasted animals palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1[n-9]), linoleic acid (18:2[n-6]) and arachidonic acid (20:4[n-6]) were the major fatty acids of the liver; in refed animals 16:0, palmitoleic acid (16:1[n-7]), 18:0, 18:1(n-9) and cis-vaccenic acid (the n-7 isomer of oleic acid) were the major fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of amino acid transport system L by amino acid availability in CHO-K1 cells. A special role for leucine

Starvation of CHO-K1 cells for leucine leads to a 3-4-fold increase in transport system L activity, without modification of transport through systems A and ASC. The concentration of leucine must be below 10 microM before the enhancement of transport can be clearly seen. To achieve low concentrations of leucine such as 10 microM, extensive dialysis of fetal calf serum was required. The enhancement of transport was completed after 12-24 h of starvation and was fully reversed within 1 h of re-feeding with leucine. Starvation for isoleucine, valine or phenylalanine also produced an increase in system L transport activity, but the effect was only one half of that seen following leucine starvation.

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.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of polyamines on cyclic AMP-mediated stimulation of amino acid transport in isolated rat hepatocytes

The effects of natural polyamines on cyclic AMP-mediated stimulation of amino acid transport in isolated rat hepatocytes were analyzed. Despite the fact that polyamines could directly compete with alpha-aminoisobutyric acid (AIB) for uptake, preincubation of hepatocytes with polyamines did not significantly alter basal AIB transport. The stimulatory effect of glucagon or cyclic AMP analogs was differently affected by polyamines, since it was reduced in the presence of spermine and, inversely, potentiated by spermidine, putrescine, and cadaverine. Dose-dependence analysis showed that half maximal and maximal effects occurred with 2-3 and 6-10 mM external concentrations, respectively. None of the polyamine effects could be ascribed to transstimulation or transinhibition of amino acid uptake. The inhibitory effect exerted by spermine correlated its capacity to inhibit [3H]-leucine incorporation into proteins partially. The potentiating effect of the other polyamines did not result from stabilization of newly synthesized carrier proteins. Instead, the increase in Vmax of the high affinity transport component suggested that more carriers became available, presumably because polyamines facilitated their synthesis by interacting directly with one or several steps controlled by cyclic AMP. Polyamines appear to represent a new class of factors capable of modulating the cyclic AMP-mediated stimulation of amino acid transport, in hepatocytes.

Absence of pyridoxine- (pyridoxamine-) 5'-phosphate oxidase in Morris hepatoma 7777

Morris hepatoma 7777 previously has been shown to have no detectable pyridoxine- (pyridoxamine-) 5'-phosphate oxidase activity [Thanassi, J. W., Nutter, L. M., Meisler, N. T., Commers, P., & Chiu, J.-F. (1981) J. Biol. Chem. 256, 3370-3375]. In order to determine if this enzyme was missing in the hepatoma, we purified rat liver oxidase and raised antibodies to it in rabbits. Final purification of rat liver oxidase for use as an antigen was accomplished by affinity chromatography and gel electrophoresis. The rat liver enzyme is similar to rabbit liver oxidase [Kazarinoff, M. N., & McCormick, D. B. (1975) J. Biol. Chem. 250, 3436-3442] having two noncovalently linked subunits with molecular weights in the range of 25 000-28 000. Evidence indicating that inactive enzyme was simultaneously purified with native enzyme was obtained. The IgG fraction was purified from the serum of a rabbit that had been immunized with rat liver oxidase. This was used in the development of ELISA and immunoblot analyses for the presence of antigenically active pyridoxine- (pyridoxamine-) 5'-phosphate oxidase in cytosolic preparations from normal rat liver and Morris hepatoma 7777. The results indicated that there was no immunologically detectable oxidase protein in the tumor. An alternate pathway of pyridoxal 5'-phosphate synthesis, involving oxidation of pyridoxine to pyridoxal followed by phosphorylation, was ruled out. The implications of these findings with respect to acquisition of nutrients by tumors are discussed.

Regulation of amino acid transport in L6 myoblasts. II. Different chemical properties of transport after amino acid deprivation

The mechanism of stimulation of amino acid transport system A caused by amino acid deprivation in L6 cells was investigated. In cells loaded with alpha-aminoisobutyric acid (AIB), amino acid deprivation increased the rate of proline uptake only after the intracellular [AIB] dropped below 7 mM. Efflux of proline was not sensitive to the presence of proline in the outer medium (with or without external Na+), suggesting that efflux through system A (and possibly uptake) is not susceptible to transinhibition. Transport (stimulated uptake) into amino acid-deprived cells and that into amino acid-supplemented cells differed in several chemical properties: 1) In the former group, transport was higher at lower pH values than in the latter, and the optimum pH values were 7.5 and 7.8, respectively. 2) Unlike proline uptake in supplemented cells, uptake in deprived cells was inhibited by 50% with N-ethylmaleimide (1 mM) or by 50 microM p-chloromercuribenzoate (PCMBS). Inhibition by PCMBS was not due to collapse of the Na+ gradient. The mercurial inhibited only the deprivation-induced stimulation of transport, bringing the rate of proline uptake to the "basal" uptake level observed in amino acid-supplemented cells. Proline uptake was not stimulated by a second deprivation following treatment with PCMBS and a supplementation-deprivation cycle. However, in untreated cells, or by reversing mercaptide formation with dithiotreitol, the second deprivation stimulated transport. Deprivation at 4 degrees C did not elicit stimulation of proline uptake. Cycloheximide prevented the stimulation and decreased the rate of proline uptake in deprived cells more efficiently than in supplemented cells. Actinomycin D prevented stimulation when added at the onset of deprivation. The above data indicate that stimulation of transport by deprivation is protein synthesis-dependent and that the stimulated transport had chemical properties distinct from the "basal" transport in supplemented cells. The evidence presented is consistent with a model of activation of a finite pool of transporters upon deprivation, the chemical characteristics of which differ from those of the "basal" transport system.

Insulin and glucagon binding and stimulation of amino acid transport in isolated hepatocytes from streptozotocin diabetic rats

The binding of insulin and glucagon and the effects of these hormones on amino acid transport were examined in isolated rat hepatocytes from streptozotocin diabetic rats. Hepatocytes from diabetic rats bound more insulin than cells from control animals. These changes were accounted for by a 50%-60% increase in the number of insulin receptors per cell. Glucagon binding did not significantly differ in hepatocytes from both groups. Following a 2 hr incubation of the cells in vitro, the basal rate of alpha-aminoisobutyric acid (AIB) influx was enhanced in diabetic rat hepatocytes compared to controls. This alteration was accounted for by an increase in the Vmax of both a low affinity and a high affinity component of transport. The ability of diabetic rat hepatocytes to respond to maximally stimulating concentrations of insulin or glucagon by enhancing further the rate of AIB influx was markedly diminished. Hormone responsiveness was restored to normal in hepatocytes from insulin-treated diabetic animals. The data suggest that in diabetic rat hepatocytes the diminished insulin and glucagon responsiveness with regard to the stimulation of amino acid transport stems from postreceptor alteration(s).

Placental amino acid uptake. VI. Regulation by intracellular substrate

Amino acid uptake by human placental tissue is regulated by intracellular amino acids. alpha-Aminoisobutyric acid (AIB) uptake was reduced at intracellular AIB concentrations of 0.8 mM. The magnitude of reduction increased sharply between 1 and 3 mM and reached a maximum of 45% at 5 mM. Suppression was specific to the "A" system. It occurred only when both the amino acid used for preloading and that used as an uptake substrate were active with that system. In the "L" system, facilitation apparently occurs, and in the "ASC" system there is no apparent effect. The system specificity as well as other evidence indicated that suppression is caused by substrate present intracellularly rather than by dilution of extracellular substrate. Suppression was independent of inhibitors of protein synthesis and was not seen in membrane vesicles prepared from preloaded tissue, indicating that intracellular substrate interacts directly with the carrier (transinhibition) rather than altering its synthesis or degradation. The A system transinhibition has the potential to regulate syncytial uptake in vivo and limit variation due to changes in maternal plasma amino acid concentration.

Effects of hormones and amino acid depletion on the kinetic parameters of amino acid uptake in monolayer cultures of rat hepatocytes

Uptake of 2-aminoisobutyric acid (AIB) at concentrations of 0.1 mM to 30 mM was examined in sodium-containing and sodium-free media in hepatocytes pretreated without hormones (control), with hormones, or with amino acid depletion. Results show that 1-minute but not 4-minute rates can be taken as initial rates for the total or sodium-dependent transport of AIB. The data for the 1-minute sodium-dependent transport of AIB were analyzed by a computer program and also by Eadie-Hofstee and Lineweaver-Burk plots, and a single saturable system was found. In the control cultures, the saturable system had a Km of 1-2 mM AIB and a Vmax of 1.2 nmoles AIB/mg protein/minute. There was an increase in the Vmax of two to three-fold after pretreating the cultures with insulin or amino acid depletion, three to four-fold with glucagon, and six to seven fold with glucagon + dexamethasone.

Binding and action of insulin and glucagon in monolayer cultures and fresh suspensions of rat hepatocytes

Insulin and glucagon binding, and the subsequent stimulation of amino-acid transport, were investigated in adult-rat hepatocytes. Cells were used either in suspension shortly after isolation, or as monolayers after 20 h of culture in a serum-free medium. At 37 degrees C, hepatocytes in monolayer cultures bound 2.5 times as much insulin and glucagon as did freshly isolated cells, owing to an increase in the total number of binding sites per cell. For both hormones, these differences could be accounted for mainly by a greater number of low-affinity binding sites in primary cultured hepatocytes compared with freshly isolated cells. Exposure of hepatocytes to insulin or glucagon for 2-3 h at 37 degrees C in a medium free from amino acids increased the capacity (primary cultures) or induced the emergence (fresh suspensions) of a similar high-affinity component (Km approximately mM) of alpha-aminoisobutyric-acid (AIB) transport. Primary cultured hepatocytes were more sensitive to insulin (half-maximal effect occurred with insulin at approximately 0.3 nM) than freshly isolated cells (half-maximal effect approximately 0.7 nM) for the stimulation of AIB transport, whereas the dose-response curves were virtually indistinguishable for the glucagon stimulation of AIB transport in both preparations of cells (half-maximal effect occurred with glucagon at approximately 1.5 nM). These results indicate that, despite differences in the apparent insulin- and glucagon-binding capacities (which involved mainly a low affinity site), both freshly isolated and primary cultured (20-h monolayers) hepatocytes behave similarly in response to insulin and glucagon with regard to the stimulation of amino acid transport.

Amino acid transport in isolated rat hepatocytes

Improvements in the collagenase perfusion techniques have made isolated rat hepatocytes a popular model in which to study hepatic function. Our knowledge of hepatic amino acid transport has been advanced as a result of this methodology. Translocation across the hepatocyte plasma membrane can, in some instances, represent the rate-limiting step in the overall metabolism of certain amino acids. Furthermore, regulation of amino acid uptake by hepatocytes appears to play a role in diabetes, and perhaps in malignant transformation. Comparisons between normal adult hepatocytes and several hepatoma cell lines show basic differences in amino acid transport. There are at least eight distinct systems in normal hepatocytes for transport of the hormones. Systems A and N exhibit enhanced uptake rates after the cells have been maintained in the absence of extracellular amino acids, a phenomenon termed adaptive control. Further studies using isolated hepatocytes will increase our basic understanding of membrane transport processes and their 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.

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.

Na+-gradient-dependent transport of L-proline and analysis of its carrier system in brush-border membrane vesicles of the guinea-pig ileum

Transport of L-proline was studied with membrane vesicles prepared from the brush borders of the guinea-pig ileum. The presence of an Na+ gradient from outside to inside of the vesicles stimulated L-proline uptake. Accumulation of amino acid in the vesicles reached a maximum 30 s after incubation, then decreased due to efflux and finally equilibrated at a level nearly identical to that shown in the absence of an Na+ gradient in 30 min. The peak level of the uptake was 3.5-times greater than the final equilibrium level. The equilibrium level of L-proline uptake decreased with increasing medium osmolarity. Extrapolation to infinite medium osmolarity, that is, under the condition of zero intravesicular space, showed no uptake, indicating transport of L-proline into membrane vesicles. The initial rate of uptake for 15 s was enhanced with increasing concentrations of Na+ in the external medium. A small part of the L-proline transport occurred by simple diffusion in addition to Na+-gradient-dependent transport. When L-proline concentrations were varied and transport due to diffusion was subtracted, the initial rate of uptake dependent on Na+ gradient (out greater than in) obeyed Michaelis-Menten kinetics with Km and V values of 0.67 mM and 2.73 nmol/15 s per mg protein, respectively. Evidence was obtained which indicates that L-cysteine is a substract specific for transport through system ASC (alanine-, serine-, and cysteine-preferring) and that transport in the presence of an Li+ gradient (out > in) also takes palce by the ASC system. The uptake of L-proline in the presence of an Na+ gradient (out > in) was inhibited 90% by a large excess of alpha-(methylamino)-isobutyrate, the model substrate specific for the A system (alanine-preferring). This indicates than 90% of Na+-gradient-dependent L-proline uptake is supported by the A system. The remaining 10% of L-proline uptake was found to be catalyzed by the ASC system, since L-proline uptake equivalent to this alpha-(methylamino)-isobutyrate-uninhibited part was demonstrated in the presence of Li+ gradient.

Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

The effects of insulin, glucagon of Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2-amino [1-14C]isobutyric acid (AIB) and N-Methyl-2-amino [1-14C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium-dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane-1-carboxylic acid, 1- [carboxyl-14C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium-independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium-dependent system that does not transport NMe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2--3-fold increase; however, the addition of glucagon to cultures containing DEX caused a 7--8-fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.