Sodium-dependent amino acid transport in reconstituted membrane vesicles from Ehrlich ascites cell plasma membranes

Nucleoside transporter expression and activity is regulated during granulocytic differentiation of NB4 cells in response to all-trans-retinoic acid

NB4 cells express multiple nucleoside transporters (NTs), including: hENT1 (es), and hENT2 (ei), and the CNT subtype referred to as, csg; a concentrative sensitive guanosine specific transporter. csg activity is a distinguishing feature of the NB4 cell line and its presence suggests a particular requirement of these cells for guanosine salvage. Proliferation and differentiation pathways determine, in part, the number of NTs in cells and tissues. In this study, all-trans-retinoic acid (ATRA)-induced granulocytic differentiation of NB4 cells resulted in biphasic changes in guanosine transport. Transient increases in csg and es activity, the result of an increase in V(max) (pmol/muls) of both transporter systems, served as early markers of differentiation while expression of a fully differentiated phenotype was accompanied by a selective loss of csg activity and the return of es activity to that of proliferating cells. Intracellular incorporation of [(3)H]-guanosine decreased as cells matured despite increased transport rates and suggested a reduced intracellular requirement of NB4-granulocytes compared to their proliferating counterparts. Whether a loss of csg activity could serve to assess clinical response to differentiation therapies is not known. Nitrobenzylthioinosine (NBMPR) binding sites within nuclear membrane (NM) preparations, suggested the presence of functional intracellular NTs. An increase in plasma membrane (PM) associated transporters coincided with the early increase in guanosine transport and a decrease in NBMPR binding to NM fractions and suggests that intracellular NTs may serve as a reserve pool for translocation to the (PM) when additional transport capacity is required. The modulation of transporters during differentiation could potentially regulate drug bioavailability and cytotoxicity and should be evaluated prior to combining differentiating agents with traditional nucleoside analogs in the treatment of APL.

Revisiting the road to the discovery of exosomes

A look-back in time is presented to review the 'Alice in Blunderland' approach which led to the discovery of exosomes in and their extrusion from maturing reticulocytes. The key experiments which established that these structures are naturally occurring in both nucleated and non-nucleated reticulocytes are reviewed. The protein content of reticulocyte exosomes is largely that of plasma membrane proteins which are known to diminish in the course of reticulocyte maturation. Thus their protein content may vary depending on the species origin of the reticulocyte. To date, all exosomes also contain HSP 70, a major cellular chaperone which is not transmembrane bound. Significantly, the proteins externalized are intact and retain catalytic activity as well as native ligand binding activity. Their fate remains unknown.

All- trans-retinoic acid increases cytotoxicity of 1-beta-D-arabinofuranosylcytosine in NB4 cells

PURPOSE

Clinically, the benefits of combining all- trans-retinoic acid (ATRA) with chemotherapy have been well documented in the treatment of acute promyelocytic leukemia (APL). Changes in nucleoside transporter expression and activity have been shown to occur in NB4 cells in vitro following treatment with ATRA. In this study we investigated whether ATRA treatment increases sensitivity to ara-C in NB4 cells. Specifically, we examined the role of ATRA-associated changes in nucleoside transporter expression and activity in eliciting ara-C cytotoxicity.

METHODS

Cellular uptake of [(3)H]-ara-C and nucleoside transporter abundance were determined in untreated cells and cells treated with 1 microM ATRA for 12-72 h using an inhibitor and oil stop procedure, and an equilibrium [(3)H]-NBMPR binding assay, respectively. Cytotoxicity of ara-C and the apoptotic response prior to and following ATRA treatment were determined using the MTT viability assay and the TUNEL assay, respectively.

RESULTS

ATRA treatment increased ara-C cytotoxicity and potency, ara-C transport, and augmented ara-C-induced apoptosis. The combination effect was supraadditive under some conditions and sequence-dependent whereby the maximum effect was seen when the addition of ATRA preceded the addition of ara-C, and when ara-C administration closely followed ATRA administration.

CONCLUSIONS

The ATRA-induced increase in cytotoxicity of ara-C was, in part, the result of an increase in the functional expression of nucleoside transporters, and a role for bcl-2 was also indicated. Our results would suggest that timing of ara-C therapy should be tied to maximal es transporter expression, which is likely to be 24 h after ATRA treatment begins. It remains to be seen whether the response in the clinic can be further enhanced in APL by taking advantage of ara-C transporter regulation by ATRA.

PKC regulation of the human equilibrative nucleoside transporter, hENT1

Regulation of nucleoside transporters is poorly understood. We show that acute stimulation of protein kinase C (PKC) causes a rapid increase in S-(4-nitrobenzyl)-6-thioinosine-sensitive (human equilibrative nucleoside transporter 1, hENT1) nucleoside uptake, in human cultured cells, which is not due to increased metabolism and which can be blocked by PKC inhibitors. Use of isoform-specific inhibitors indicates that PKC delta and/or epsilon (but not alpha, beta or gamma) are responsible for the acute effects. Down-regulation of PKC decreases hENT1-dependent uridine uptake. These are the first data to show rapid PKC delta/epsilon-dependent stimulation of hENT1 transport by a mechanism that may involve activation of transporters at the membrane possibly by post-translational modification of the protein.

Demonstration of equilibrative nucleoside transporters (hENT1 and hENT2) in nuclear envelopes of cultured human choriocarcinoma (BeWo) cells by functional reconstitution in proteoliposomes

The equilibrative nucleoside transporters (ENTs) are a newly recognized family of membrane proteins of which hENT1 is the nitrobenzylmercaptopurine ribonucleoside (NBMPR)-sensitive (es) and hENT2 the NBMPR-insensitive (ei) transporter of human cells. BeWo cells exhibit large numbers (>10(7)/cell) of NBMPR-binding sites and high es and ei nucleoside transport activities relative to other cell types. In this work, we have demonstrated that proliferating BeWo cells possess (i) mRNA encoding hENT1 and hENT2 and (ii) hENT1-specific immunoepitopes. We examined NBMPR binding and its inhibition of uridine transport in various BeWo membrane fractions and proteoliposomes derived therefrom to determine if NBMPR binding to intracellular membranes represented interaction with functional es transporters. Unfractionated membranes and fractions enriched 5-fold in plasma membranes relative to postnuclear supernatants exhibited high NBMPR binding activity. Intact nuclei and nuclear envelopes also exhibited abundant quantities of NBMPR-binding sites with affinities similar to those of enriched plasma membranes (Kd = 0.4-0.9 nM). When proteoliposomes were made from octyl glucoside-solubilized membranes, high affinity NBMPR-binding sites were not only observed in crude membrane preparations and plasma membrane-enriched fractions but also in nuclear envelope fractions. Proteoliposomes prepared from either unfractionated membranes or nuclear envelopes exhibited both hENT1-mediated (82-85%) and hENT2-mediated (15-18%) transport of [3H]uridine. These results provided evidence for the presence of functional es and ei transporters in nuclear membranes and endoplasmic reticulum, suggesting that hENT1 and hENT2 may function in the translocation of nucleosides between the cytosol and the luminal compartments of one or both of these membrane types.

Is gamma-actin a regulator of amino acid transport?

A mutated yeast cell line incapable of growth in minimal medium with proline as the sole nitrogen source was restored to normal growth by transfection with a cDNA from mouse Ehrlich cells. The cloned cDNA (E51) was found to be 90% homologous to gamma-actin. Immediately after transfection with E51 cDNA, both alpha-aminoisobutyric acid (AIB) and proline uptake in the mutated yeast were increased, particularly at pH 5. The expression of the same E51 cDNA also enhanced amino acid uptake in Xenopus laevis oocytes after injection into the Xenopus nuclei. A mutated mammalian lymphocyte cell line (GF-17), deficient in system A transport, also showed increased Na(+)-dependent transport after transfection with E51 cDNA. Whereas the mock transfected GF-17 cells failed to grow in the selection medium, the transfectants with E51 cDNA grew better than the untransfected cells. The data are consistent with the conclusion that expression of E51 cDNA can modify inactive, endogenous amino acid transporters, permitting substantial amino acid uptake in cells deficient in amino acid transporter(s) and permitting rapid cell growth. The data suggest that the gamma-actin-like protein coded for by E51 cDNA may play a significant regulatory role in amino acid transport.

Sodium-dependent L-serine transport in plasma membrane vesicles isolated from Ehrlich cells by two-phase compartmentation

Plasma membrane vesicles were prepared from Ehrlich cells using two-phase system compartmentation. The highly pure plasma membrane vesicles obtained presented a negligible mitochondrial contamination and were suitable for studies of amino acid transport. L-Serine transport showed a clear ionic specificity, maximum incorporation being observed when an inwardly directed NaSCN gradient was used. Na(+)-dependent L-serine transport was dependent on assay temperature and membrane potential, and it seemed to be carried out by two different transport systems. An essential sulfhydryl group seemed to be involved in the transport process.

L-glutamine transport in native vesicles isolated from Ehrlich ascites tumor cell membranes

Native vesicles isolated from Ehrlich ascites tumor cells accumulate glutamine by means of Na(+)-dependent transport systems; thiocyanate seems to be the more effective anion. The apparent affinity constant for the process was 0.38 mM. The Arrhenius plot gave an apparent activation energy of 12.3 kJ/mol. The structural analogs of glutamine, acivicin (2.5 mM) and azaserine (2.5 mM), inhibited the net uptake by 67 and 70%, respectively. The sulfhydryl reagents mersalyl, PCMBS, NEM, and DTNB also inhibited net uptake, suggesting that sulfhydryl groups may be involved in the activity of the carrier protein. A strong inhibition was detected when the vesicles were incubated in the presence of alanine, cysteine, or serine; in addition, histidine, but not glutamate or leucine, had a negative effect on glutamine transport.

Asymmetric reconstitution of the glucose transporter from Ehrlich ascites cell plasma membrane: role of alkali cations

Gel chromatography of solubilized Ehrlich cell plasma membranes and preformed asolectin vesicles coupled to a freeze-thaw cycle results in the reconstitution of 3-O-methyl-D-glucose transport. The transport activity of the liposomes formed is critically dependent on the cation present during reconstitution. Liposomes formed in K+ show high levels of carrier-mediated 3-O-methyl-D-glucose uptake (495 pmol/min/mg protein) while those formed in Na+ do not (33 pmol/min/mg protein). The inactivity in Na+ is not due to a diminished incorporation of glucose transporter nor is it due to carrier molecules reconstituted with a different orientation from those in K+ liposomes. Instead, the low glucose transport level in Na+ liposomes is related to the small size of vesicles formed with Na+. A second freeze-thaw cycle in K+ causes a two- to threefold increase in the available intravesicular volume of Na+ liposomes and results in an eightfold increase in carrier-mediated 3-O-methyl-D-glucose uptake. K+ liposomes, treated in an identical manner, show only a twofold increase in uptake. The glucose transporter was identified as a protein with a molecular mass range of 44.7 to 66.8 kDa, by the D-glucose-inhibitable photoincorporation of [3H]cytochalasin B. The carrier protein is inserted in reconstituted vesicles in a nonrandom manner with at least 80% of the molecules oriented with the cytoplasmic domain accessible to the external medium. In contrast, the neutral Na+-dependent amino acid transport system appears to be randomly reconstituted.

Characterization of murine-specific leukemia virus receptor from L cells

The host cell receptor for Moloney murine leukemia virus was solubilized from murine L-cell membranes and characterized. In initial studies designed to identify a receptor-rich cell line, different mouse cells were screened for binding to Moloney gp70, the viral envelope glycoprotein which determines host cell-binding specificity. gp70 binding to murine L cells was specific and saturable, with an apparent affinity constant (Ka) of 4 X 10(8) M-1, and the number of receptors per cell (6 X 10(5)) was similar to that of other mouse fibroblast cell lines. Characterization of the gp70 receptor with regard to extraction by detergents, protease sensitivity, and heat denaturation suggests that the receptor is an intrinsic membrane protein. Upon extraction of L-cell membranes with 0.2% deoxycholic acid and precipitation with acetone, specific and saturable binding of gp70 could be detected. The solubilized gp70-binding component was eluted upon gel filtration on Sephacryl S-300 into a species with an approximate molecular weight of 110,000.

Reconstitution of the melibiose carrier of Escherichia coli

Different conditions were studied for optimal solubilization and reconstitution of the melibiose carrier of Escherichia coli. Several alpha- and beta-galactosides, known to be substrates for the melibiose carrier, were found to inhibit [3H]-melibiose uptake by proteoliposomes. In the presence of 10 mM Na+ the Km for melibiose counterflow was 0.42 mM. Melibiose and raffinose were good substrates for counterflow, while thiomethyl-beta-galactoside and p-nitrophenyl-alpha-galactoside were accumulated very poorly. Although the latter two sugars are known to be substrates for the carrier, they showed a very rapid rate of passive diffusion across the liposome membrane. The proton ionophore carbonylcyanidechlorophenylhydrazone had no effect on uptake, suggesting that a proton motive force is not essential for the counterflow phenomenon.

A simple and efficient method for reconstitution of amino acid and glucose transport systems from Ehrlich ascites cells

Solubilized Ehrlich cell plasma membrane proteins were incorporated into lipid vesicles in the presence of added phospholipid, using Sephadex G-50 chromatography combined with a freeze-thaw step. Liposomes formed in K+ exhibited high levels of Na+-dependent, alpha-aminoisobutyric acid uptake which was electrogenic and inhibited by other amino acids. The transport activity reconstituted was similar to that observed in native plasma membrane vesicles. In addition to transport by system A, leucine exchange activity (system L), Na+-dependent serine exchange activity (system ASC), and stereospecific glucose transport activity were also reconstituted. The latter was inhibited by D-glucose, D-galactose, cytochalasin B, and mercuric chloride. The medium used for reconstitution was critical for the recovery of Na+-dependent amino acid transport. The use of Na+ in the reconstitution procedure led to formation of liposomes which displayed little Na+-dependent and gradient-stimulated amino acid uptake. In contrast, all transport activities studied were efficiently reconstituted in K+ medium.

Partial purification and reconstitution of the aspartate transport system from Halobacterium halobium

Membrane vesicles of Halobacterium halobium R1Wrm bind to an aspartic acid-agarose affinity column. After disruption of the bound vesicles by low ionic strength, a protein fraction is eluted from the column with 2.5% cholate in 3 M NaCl. When this fraction is reconstituted with soybean lipids to form proteoliposomes, the proteoliposomes exhibit active aspartate accumulation. Aspartate transport in the reconstituted system is driven by a chemical sodium gradient (out greater than in), exhibits sensitivity to an electrical potential, and is specific for L-aspartate. These characteristics are consistent with observations on aspartate transport in intact membrane vesicles of H. halobium. Initial aspartate transport rates in the reconstituted system are about ninefold enhanced over the native system. The system developed should be useful in future purification schemes and studies of the molecular details of membrane transport.

Partial purification of amino acid transport systems in Ehrlich ascites tumor cell plasma membranes

Ehrlich ascites tumor cell plasma membranes were subjected to sequential selective protein extraction to identify protein components associated with amino acid transport. These membranes were extracted with Triton X-100 followed by 2,3-dimethylmaleic anhydride. Approximately 80% of the membrane proteins were extracted by these procedures while the original lipids were largely retained (approximately 70%). The quantity of carbohydrate per milligram protein in the residue increased on extraction, consistent with an enrichment of glycoprotein in the residue. The residual vesicles display the characteristic properties of Na+-coupled amino acid transport. These properties include Na+-stimulated uptake and Na+-gradient-stimulated uptake leading to an accumulation of the solute against its chemical gradient as well as inhibition of uptake by a competitive amino acid, L-methionine. The extracted vesicles exhibit a peak level of alpha-aminoisobutyrate uptake six times greater than that expected from equilibration of alpha-aminoisobutyrate. This accumulation is greater than that obtained with native vesicles, albeit slower. The accelerated exchange diffusion of L-leucine is not measurable in the residual vesicles after dimethylmaleic acid anhydride treatment, although it can be measured after Triton extraction. These results are consistent with the conclusion that the amino acid transport systems "A" (Na+-coupled) and "L" (Na+-independent) in Ehrlich cells, though having overlapping specificities for amino acids, and distinct physical entities.

The involvement of the membrane oxidoreduction system in stimulating amino acid uptake in Ehrlich ascites tumor cells

The addition of 5 mM ascorbate plus 0.09 mM phenazine methosulfate stimulated 2- to 3-fold the initial rate of 2-aminoisobutyric acid transport into Ehrlich cells. This was observed under the conditions in which glycolysis and mitochondrial electron transport were blocked by iodoacetate and KCN, and the cellular ATP level was maintained below 0.1 mM. Proton conductors, carbonylcyanide m-chlorophenylhydrazone and SF6847 did not affect the stimulation of 2-aminoisobutyric acid uptake caused by ascorbate plus phenazine methosulfate. Ascorbate was replaced by NADH but not by NADPH, and phenazine methosulfate was the only effective acceptor in stimulating 2-aminoisobutyric acid uptake. The stimulating effect of ascorbate plus phenazine methosulfate was due to an increase in the V value for 2-aminoisobutyric acid but not in the Km value. This effect required the presence of an Na+ gradient and was accompanied by an increase in 22Na+ influx. The molar ratio of 2-aminoisobutyric acid to Na+ uptake enhanced by ascorbate plus phenazine methosulfate was calculated to be 1 : 1. Quinacrine, an inhibitor of NADH oxidoreductase in the plasma membrane, inhibited both the enhanced rate of 2-aminoisobutyric acid and Na+ transport without affecting the basal transport activity. The stimulatory effect of ascorbate plus phenazine methosulfate was also observed with other amino acids, alanine, glycine, proline and cycloleucine which are known to be transported via an Na+-dependent system but not with leucine and threonine. These results suggest that a redox system in the plasma membrane participates in energy coupling for amino acid transport by increasing the rate of cotransport with Na+.

Solubilizaiton of the choline transport system and re-incorporation into artificial membranes

The carrier-mediated transport of choline has been described in several types of cell and at isolated synaptic terminals. The operation of the caarrier seems to be a rate-controlling factor for the synthesis of acetylcholine at the synaptic terminals that release this neurotransmitter. The molecular mechanism by which the carrier transports choline is unknown and difficult to investigate because at present it can only be studied in the complex environment of the natural membrane in which it is situated. Furthermore, such studies are hampered by the results of metabolic activity inside the cell itself. The resolution of these difficulties by the separation, purification and examination of carriers in defined systems is hindered by the problem of assaying its activity during the purification procedure. We describe here the separation of the choline carrier from the membrane and its re-incorporation into unilamellar liposomes with retention of its functional activity, thus enabling its purification and examination in isolation to be undertaken.

Active Ca2+ transport by vesicles reconstituted from Triton X-100-solubilized pigeon erythrocyte membrane

Pigeon erythrocyte membrane was solubilized partially, but relatively unselectively by Triton X-100. Vesicles were reconstituted from mixtures of Triton-solubilized membrane and lipid (phosphatidylcholine plus phosphatidyl-ethanolamine plus cholesterol) by addition of bovine high-density lipoprotein. This efficiently removed the Triton X-100. Sodium dodecyl sulfate-polyacrylamide gel electropherograms of reconstituted vesicles showed band patterns resembling those of the original membrane. The reconstituted vesicles showed ATP-dependent active accumulation of 45Ca2+. ATP-dependent 45Ca2+ uptake by the reconstituted vesicles resembled the corresponding activity of the original membrane vesicles; in both preparations the Ca2+ uptake rate depended on the square of the Ca2+ concentration and had similar [Ca2+]1/2 values, 0.16 microM and 0.18 microM, respectively.