Neutral amino acid transport in surface membrane vesicles isolated from mouse fibroblasts: intrinsic and extrinsic models of regulation

Membrane potential of rat hepatoma cells in culture: influence of factors affecting amino acid transport

The effect has been studied of various media, hormones and of amino acids on the membrane potential of rat hepatoma cells in culture measured by microelectrode impalement. Cells in Eagle's minimal essential medium plus 5% serum had a value which varied daily from about 5-8 mV, inside negative. The membrane potential of rat hepatocytes was measured to be 8.7 +/- 0.2 mV, inside negative. The membrane potential of the hepatoma cells was decreased by insulin and increased by glucagon. Membrane potential was unaffected by change of medium to Hanks' or Earle's balanced salt solutions or deprivation of serum. It was, however, reduced in cells in phosphate-buffered saline and by reduction of pH. The former effect was shown to be due to the higher [Na+] of phosphate-buffered saline as opposed to the other media. Addition of alanine, glycine, serine, proline and methylaminoisobutyrate all reduced membrane potential by 2-3 mV. Smaller decreases were seen with methionine, leucine and phenylalanine, but none with glutamine, threonine, BCH (2-aminonorborane-2-carboxylic acid) and D-alanine. The results are compared with the effects of similar conditions on aminoisobutyrate uptake. Whilst there was a correlation under some conditions there was not under others. It is concluded that for the hepatoma cells factors additional to the membrane potential must exert some influence on the capacity for amino acid transport.

Detection and separation of human red cells with different calcium contents following uniform calcium permeabilization

1. The human red cell, permeabilized to calcium with the ionophore A23187, is extensively used to study Ca2+ transport and the effects of intracellular Ca2+ on transport and metabolism. The interpretation of results with calcium-permeabilized cells, in general, has depended on the implicit assumption that the ionophore-induced calcium distribution among the cells is uniform. 2. To establish whether or not calcium permeabilization with the ionophore A23187 generated a uniform calcium distribution in normal-ATP red cells, a method was developed to detect and separate calcium-permeabilized red cells with different calcium contents. For the method to uncover pre-existing heterogeneity without itself inducing it, it was essential to preserve the calcium distribution which existed at the time of sampling. The method was based (i) on the ability of cytoplasmic Ca2+ to activate K+-selective channels in the membrane, and (ii) on the demonstration here that thiocyanate (SCN-) is a non-limiting co-ion for rapid net KSCN efflux and cell shrinkage in the cold. 3. Calcium-permeabilized cells in pump-leak steady state were washed free of ionophore using ice-cold, albumin-containing media. Subsequent incubation at 0 degrees C in low-K+ media with 45-75 mM-SCN- generated dense-cell fractions (H cells) in less than 10 min. These could be separated from the remaining light cells (L cells) by either centrifugation over phthalate oils, or differential osmotic haemolysis, with conservation of the mean total cell calcium. 4. Analysis of the calcium content of H and L cell fractions revealed striking differences in their calcium content, with 70-99% of the mean total cell calcium in the H cell fraction. 5. The ionophore content of density-separated cells, processed with omission of the ionophore removal step, was similar for cells with high- and low-calcium. Magnesium loss from ionophore-treated red cells suspended in magnesium-free media followed single exponentials. Thus ionophore distribution and induced permeability were uniform, and the unequal cell calcium content must be due to factors affecting active calcium extrusion.

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.

Two membrane-bound proteins associated with alanine resistance and increased A-system amino acid transport in mutants of CHO-K1

Growth of CHO-K1, a proline auxotroph, is inhibited by amino acids that prevent proline transport. From a hydroxyurea-treated, alanine-resistant, constitutive mutant, alar4, we isolated, in a stepwise fashion, mutants, resistant to higher concentrations of alanine, that have increased velocity of amino acid transport through the A system. Two such mutants, alar4-H2.1 and alar4-H3.9, isolated as resistant to 50 mM and 125 mM alanine, respectively, showed increases in Vmax of proline transport through the A system that are directly proportional to their resistance to alanine. Alar4-H3.9, as compared to alar4 and CHO-K1, has six and 29 times the Vmax of proline transport through the A system and two and five times the velocity of transport through the combined ASC and P systems, respectively, and no change in system L. No double-minute or homologous staining regions were detectable in alar4-H3.9. A-system activity of alar4-H2.1 and alar4-H3.9, when grown under nonselective conditions, was stable for 20 generations and then declined. The phenotype of alar4-H3.9 is codominant with that of alar4 and partially recessive to that of CHO-K1. Membrane vesicles prepared from alar4-H3.9 show increases mainly in A-system transport. In sodium dodecylsulfate-polyacrylamide gel electrophoresis analysis of A-system active membrane vesicles and endoplasmic reticulum, two bands of molecular weight of approximately 62-66 kd and 29 kd are present in higher concentrations in alar4-H3.9 than in CHO-K1. These results are compatible with the hypothesis that the phenotype of alar4-H3.9 is the result of gene amplification of an A-system transporter structural gene and that the two bands may represent this transporter.

Differential effects of transmembrane potential on two Na+-dependent transport systems for neutral amino acids

The effects of changes of membrane potential on amino acid transport through systems A, ASC and L was investigated in the Ehrlich cell and the human erythrocyte. Changes of membrane potential were produced by incubating cells whose K+ permeability had been increased, either by valinomycin or by activation of Ca2+-dependent K+ channels, in medium containing different K+ concentrations. The changes in membrane potential were followed by measuring the distribution ratio reached by lipophilic indicators. Transport through Na+-dependent system A was sensitive to the membrane potential, the rate of amino acid uptake increasing 2.2-3.1-times for each 60 mV-hyperpolarization. The Na+-dependent system ASC was insensitive to membrane potential. The Na+-independent system L was not directly affected by membrane potential, but the steady-state accumulation of system L substrates was increased by hyperpolarization.

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.

Na+ -dependent proline transport in isolated membrane vesicles from the L6 muscle cell line. Stimulation of uptake by intravesicular proline

Membrane vesicles of L6 myoblasts were prepared in order to study the amino acid transport system A. The role of the membrane in the adaptive response of transport to amino acid-supplementation was assessed. The membranes, prepared by N2 cavitation, displayed Na+ (but not K+)-dependent L-proline uptake. An overshoot of L-[3H]proline uptake was observed after exposure of the vesicles to an inward Na+ gradient. Isolated membrane vesicles loaded with 50 microM proline displayed countertransport (stimulation of proline uptake). It is concluded that the adaptive decrease of proline uptake observed in amino acid-supplemented cells cannot be accounted for by trans-inhibition of transport.

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.

Distinct mechanisms of hypoxanthine and inosine transport in membrane vesicles isolated from Chinese hamster ovary and Balb 3T3 cells

Both enzyme-mediated group translocation and facilitated diffusion have been proposed as mechanisms by which mammalian cells take up purine bases and nucleosides. We have investigated the mechanisms for hypoxanthine and inosine transport by using membrane vesicles from Chinese hamster ovary cells (CHO), Balb/c 3T3 and SV3T3 cells prepared by identical procedures. Uptake mechanisms were characterized by analyzing intravesicular contents, determining which substrates could exchange with the transport products, assaying for hypoxanthine phosphoribosyltransferase activity, and measuring the stimulation of uptake of hypoxanthine by phosphoribosyl pyrophosphate (PRib-PP). We found that the uptake of hypoxanthine in Balb 3T3 vesicles was stimulated 3--4-fold by PRib-PP. The intravesicular product was predominantly IMP. The hypoxanthine phosphoribosyltransferase activity copurified with the vesicle preparation. These results suggest the possible involvement of this enzyme in hypoxanthine uptake in 3T3 vesicles. In contrast to the 3T3 vesicles, CHO vesicles prepared under identical procedures did not retain hypoxanthine phosphoribosyltransferase activity and did not demonstrate PRib-PP-stimulated hypoxanthine uptake. The intravesicular product of hypoxanthine uptake in CHO vesicles was hypoxanthine. These results and data from our kinetic and exchange studies indicated that CHO vesicles transport hypoxanthine via facilitated diffusion. An analogous situation was observed for inosine uptake; CHO vesicles accumulated inosine via a facilitated diffusion mechanism, while in the same experiments SV3T3 vesicles exhibited a purine nucleoside phosphorylase-dependent translocation of the ribose moiety of inosine. Vesicles prepared from a CHO cell line temperature-sensitive for hypoxanthine uptake (Azarts) showed a temperature-sensitivity in Km for uptake parallel to that of the intact cells. This suggests that the defect in Azarts may be caused by a missense mutation in the gene coding for the hypoxanthine transport carrier.

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.

Amino acid and glucose transport in sarcolemmal vesicles from chick embryo heart

Purified plasma membranes from chick embryo heart are shown to retain several functional properties of intact cardiac cells. (1) Muscarinic cholinergic receptors copurify with previously used cell-surface markers, i.e., (K+-dependent) p-nitrophenylphosphatase and insulin receptors (Paris, S., Fosset, M., Samuel, D. and Ailhaud, G. (1977) J. Mol. Cell. Cardiol. 9, 161-174). (2) Neutral amino acids (L-alanine and alpha-aminoisobutyric acid) are actively transported into these osmotically active vesicles when an Na+ electrochemical gradient is imposed. The affinity and specificity for amino acids are similar to those described for intact cardiac cells. (3) D-Glucose is taken up more rapidly than L-glucose. The sterospecific transport system is saturable and Na+-independent. The Km value is close to that observed in intact cells with glucose analogues.

Amino acid uptake by Saccharomyces cerevisiae plasma membrane vesicles

A procedure is described which allows for the efficient separation of Saccharomyces cerevisiae plasma membranes from other cellular membranes by discontinuous sucrose density gradient centrifugation. After vesiculization in an osmotic stabilization buffer the plasma membrane vesicles retain the ability to transport amino acids. Amino acid uptake was affected by the proton gradient dissipator m-chlorocarbonylcyanide phenylhydrazone and was dependent, in some cases, on the presence of sodium ion.

The use of membrane vesicles in transport studies

Transport-competent plasma membrane vesicles isolated from mammalian cells provide a system to investigate mechanisms and regulation of nutrient and ion transport systems. The characteristics of membrane vesicle systems to study transport in erythrocytes, renal and epithelial membranes, Ehrlich ascites cells, and mouse fibroblasts are discussed. Studies of Na+-stimulated and Na+-independent amino acid and glucose transport in these systems are evaluated, with emphasis on experimental verification of concepts stated in the Na+ gradient hypothesis. Nucleoside, phosphate, and calcium transport systems in plasma membrane vesicles from mouse fibroblast cultures are discussed. Also, current biochemical approaches to investigate mechanisms of regulation of nutrient transport systems by hormones or cellular proliferative state are described.

Inhibition of alpha-aminoisobutyric acid transport in membrane vesicles from mouse fibroblasts after phosphorylation by cyclic AMP-dependent protein kinase

Cyclic AMP-dependent protein kinases from several mammalian sources inhibit Na+-dependent alpha-aminoisobutyric acid transport by membrane vesicles isolated from 3T3 cells. Evidence is provided that phosphorylation of membrane proteins by the enzyme is responsible for the inhibition. Lysis of the vesicles, or a reduction in the intravesicular volume is not the cause of reduced transport. The cyclic AMP-dependent protein kinase and its catalytic subunit phosphorylate a number of membrane proteins. Most of these proteins are phosphorylated, but to a lesser extent in the absence of protein kinase or cyclic AMP. The phosphorylated proteins remain associated with the membranes during hypotonic lysis treatments, which would be expected to release intravesicular contents and loosely associated membrane proteins. 32P-labeled bands detected on sodium dodecyl sulfate polyacrylamide gels after phosphorylation of membranes by the catalytic subunit of the cyclic AMP-dependent kinase are eliminated by treatment with either pronase or 1 N NaOH, but not by ribonuclease nor by phospholipase C. The stability of the incorporated radioactivity to hot acid and hydroxylamine relative to hot base suggests that most of the 32P from [gamma-32P]ATP is incorporated into protein phosphomonoester linkages.