Effect of exogenous ATP on the volume of TA3 ascites tumor cells

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Extracellular ATP stimulates norepinephrine uptake in PC12 cells

This study examined the effects of extracellular ATP on norepinephrine (NE) uptake, using PC12 cells as a model of noradrenergic neurons. Previous experiments with synaptosomes led to the hypothesis that extracellular ATP can regulate NE uptake via an ecto-protein kinase. In the present study, we examined the high-affinity uptake of NE (referred to as uptake 1) in PC12 cells in the presence of varying concentrations of extracellular ATP. In the presence of Ca2+, low concentrations of ATP (0.1 microM) increased uptake 1 by approximately 36%. This increase could be mimicked by adenosine-5'-O-(3-thiotriphosphate) tetralithium salt (ATP gamma S), an analogue of ATP which can be utilized by protein kinases, and not by 5'-adenylylimidodiphosphate tetralithium salt, a nonhydrolyzable analogue of ATP, GTP, ADP, and adenosine also had no effect on uptake 1. Preincubation of the cells with NE and ATP gamma S, followed by washing and assaying NE uptake 30 min later, resulted in a persistent increase in uptake 1. Similar pretreatment with ATP did not show this increase; however, simultaneous pretreatment with ATP and ATP gamma S blocked the activation produced by ATP gamma S alone. Kinetic analysis showed that ATP gamma S pretreatment produces an increase in the Vmax of uptake 1 without altering the apparent Km for NE. These results support the hypothesis that extracellular ATP can regulate NE uptake via an ecto-protein kinase.

Control of membrane permeability by external ATP in mammalian cells: isolation of an ATP-resistant variant from Chinese hamster ovary cells

External ATP causes a great increase in the passive permeability of the plasma membrane for phosphorylated metabolites and other small molecules in cultured mammalian cells. We previously demonstrated that in CHO-K1 cells an ATP-dependent permeability change was induced in the presence of a mitochondrial inhibitor (KCN or rotenone), a cytoskeleton-attacking agent (vinblastine) and a calmodulin antagonist (trifluoperazine). These permeability changes were reversible but long exposure, for 30-60 min, to ATP together with a mitochondrial inhibitor significantly reduced the cell viability of the treated cells. Since this cell lysis was shown to be due to the ATP-dependent permeability change, we could isolate several clones resistant to the action of the external ATP from CHO-K1 cells after repeated treatment with ATP and rotenone. In 9.1 cells, one of the isolated clones, little or no ATP-dependent permeability change was observed in the presence of either a mitochondrial inhibitor, vinblastine or trifluoperazine. This CHO variant could be specifically resistant as to the change in membrane permeability induced by external ATP, since the permeabilities for the 2-deoxyglucose and drugs used in the present studies were similar to those in the case of the parent cells. These results suggest that a specific defect or alteration in the plasma membrane is involved in the ATP-dependent permeability change. It is also reported that Mg2+-dependent ATPase activity was found on the cell surface of both CHO-K1 and 9.1 cells, and this activity was shown to be not involved in the permeability change controlled by external ATP.

ATP analogues induce membrane permeabilization in transformed mouse fibroblasts

The mechanism underlying ATP-induced permeabilization of transformed mouse fibroblasts was studied by using nonhydrolyzable analogues of ATP. Incubation of 3T6 cells with 0.6 mM of either ATP, 5'-adenylyl imidodiphosphate (p[NH]ppA) or adenosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppA) resulted in an increase of 17-, 8- or 5-times, respectively, in the cell membrane permeability, measured by the efflux of normally impermeant metabolites from the cells. The induced cell permeabilization was preceded by a reduction in the membrane potential (delta psi), determined according to the distribution of the cation tetraphenylphosphonium (TPP+) between the cells and the medium. Reduction of 26, 18 and 13 mV in delta psi was exerted by 0.6 mM of either ATP, p[NH]ppA or p[CH2]ppA, respectively. In 3T3 cells the untransformed counterparts of 3T6 cells, neither reduction of delta psi, nor alterations in membrane permeability were exerted by either ATP or by its analogues. The data indicate that the dissociation of the beta, gamma-phosphate bond is not essential for membrane permeabilization by external ATP, implying that the binding of ATP to the cell surface of transformed cells is sufficient to initiate the permeabilization process. The data also suggest that delta psi is involved in the control of membrane permeability.

Permeability change in transformed mouse fibroblasts caused by ionophores, and its relationship to membrane permeabilization by exogenous ATP

Electrogenic ionophores have been found to induce membrane permeabilization in Swiss mouse 3T3 cells that had undergone spontaneous transformation (3T6 cells). Cells attached to plastic dishes were loaded with [3H] uridine, and then the medium was replaced by buffered salt solution at pH 7.8. The enhancement of membrane permeability was assayed by following the efflux of uridine nucleotides, normally impermeant substances. Titration with electrogenic ionophores, such as carbonylcyanide m-chlorophenylhydrazone (CCCP), SF-6847 and gramicidin D, markedly increased the membrane permeability within a very narrow range of ionophore concentration. Non-electrogenic ionophores, such as monensin and nigericin, did not affect membrane permeability. Measurements of the distribution of the lipophilic cation tetraphenylphosphonium (TPP+) between the cells and their environment implied that the remarkable increase in permeability took place within a narrow range of membrane potential (delta psi). The data could be explained by a delta psi threshold value, under which aqueous channels are opened in the plasma membrane. The effects exerted by electrogenic ionophores on the plasma membrane were found to be similar to those induced by exogenous ATP. In both cases rapid efflux of K+, influx of Na+ and reduction of delta psi preceded membrane permeabilization to low molecular weight, charged molecules, such as nucleotides. It is suggested that dissipation of delta psi induces conformational alterations in membranal components, and/or topological changes, such as aggregation of protein molecules, to form membranal aqueous channels. Electrogenic ionophores permeabilize both normal (3T3) and transformed (3T6) mouse fibroblasts, whereas ATP effects are specific for transformed cells. Thus, it is postulated that ATP acts via specific sites on the surface of transformed cells.

The contractile behaviour of EGTA- and detergent-treated heart muscle

Tension responses of rat ventricular trabeculae subjected to successive 'treatment' with EGTA and Triton X-100 are described in order to investigate the effects of chemical 'skinning' techniques. In some preparations the alkaloid saponin was also used before Triton. Ultrastructural evidence is cited that the 'EGTA-treatment' fails to render cells 'hyperpermeable', i.e. freely permeable to small ions, whereas both saponin and Triton do so. In this paper we show that contractile responses like those described previously for the 'EGTA-treated' tissue can be obtained. However, more detailed examination shows that such behaviour is quantitatively distinct from that of conventionally skinned fibres in a way that is incompatible with the notion of 'hyperpermeability'. The Ca-sensitivity after treatment with either EGTA, saponin or Triton is identical in our hands. However, this is not explained by free access of Ca (and EGTA) to the intracellular space in the EGTA-treated preparation: contractures develop with very different time courses, being fastest after Triton and only marginally slower when first exposed to saponin but a factor of five times slower after 'EGTA-treatment' alone. This applies to contractures evoked direct from Ca2+ concentration congruent to 10(-9) M to the test Ca2+ concentration at constant total buffer concentration. 'EGTA-treated' fibres develop tension when ATP or creatine phosphate (CrP) are removed from the bath. However, responses to ADP and to CrP changes persist with millimolar levels of ATP present, quite unlike the Triton-skinned muscle. Exposure to each of a variety of solutions for 24 h produce preparations showing similar behaviour: whatever the explanation for the EGTA-'skinning' phenomenon it is not dependent upon low bathing Ca2+ concentration. On the basis of the functional characteristics described here, and the structural results cited, we conclude that the cell membrane continues to function as a selective permeability barrier after 'EGTA-treatment': this treatment does not produce a model of a selectively 'skinned' cardiac cell.

Cartilage nucleoside triphosphate pyrophosphohydrolase. II. Role in extracellular pyrophosphate generation and nucleotide metabolism

Extracellular generation of inorganic pyrophosphate (PPi) in cartilage organ culture is markedly augmented by ATP.ATP, not an ATP metabolite (ADP, AMP, adenosine) is necessary for this augmentation. Excess PPi production is effectively blocked by known inhibitors of nucleoside triphosphate (NTP) pyrophosphohydrolase (EDTA, EGTA, dithiothreitol). Excess 32P-PPi is generated directly from gamma 32P-ATP by cartilage, as substrate and product have similar specific activities. These findings strongly favor ecto-NTP pyrophosphohydrolase as the source of extracellular PPi generation in the presence of NTP. Additionally, active nucleotide and nucleoside catabolism is demonstrated in these cartilage organ cultures.

Protein kinase activity on the cell surface of a macrophage-like cell line, J774.1 cells

Protein kinase activity was demonstrated on the cell surface of a murine macrophage-like cell line, J774.1 cells, and was characterized in detail. When intact cells were incubated with [gamma-32P]ATP, a transfer of [32P]phosphate into acid-insoluble materials of the cells occurred. This reaction was Mg2+-dependent but cAMP-independent, and Mg2+ could be substituted for by Mn2+. The reaction products were found to be proteins, as revealed by SDS-polyacrylamide gel electrophoresis and autoradiography, with phosphomonester linkages to serine and threonine residues, but not to tyrosine. The results of experiments with chemical and enzymatic treatments as well as Con A-Sepharose column chromatography ruled out the possibility that an acyl-phosphate linkage or phosphomannosylglycopeptide was present in the reaction products. The protein kinase(s) and the reaction products were located on the cell surface of the cells, as shown by the fact that the products were removed by mild trypsinization of cells carefully controlled so that the cells remained in an intact state. Phosphorylation of exogenous proteins (phosvitin and casein) by intact cells further supported the location of the enzyme. The phosphorylated proteins of the cells were found to be metabolically stable and remained on the cell surface even at 120 min after the phosphorylation reaction. Possible roles of ecto-protein kinase activity in macrophage functions and macrophage-activation are also discussed.

Phosphorylation events during Müllerian duct regression

Regression of the fetal rat Müllerian duct in vitro was stimulated by sodium fluoride in the absence of Müllerian inhibiting substance. The action of Müllerian inhibiting substance was inhibited by sodium vanadate, adenosine 5'-triphosphate, and several related nucleotides in the presence of manganese ions. Epidermal growth factor specifically inhibited the substance, but only with manganese ions present. Insulin, platelet-derived growth factor, and nerve growth factor had no effect. These results suggest that dephosphorylation of membrane proteins mediates the action of Müllerian inhibiting substance.

On the mechanisms of ATP-induced and succinate-induced redistribution of cations in isolated rat liver cells

1. The ability of external ATP to induce calcium uptake in isolated rat liver cells was further characterized. Stimulation of calcium uptake was specific for ATP, other nucleotides or ATP metabolites had no comparable effect. ATP was dephosphorylated while stimulating calcium uptake, but there was no stoichiometry between ATP hydrolysis and calcium uptake nor did dephosphorylation depend on calcium concentration. ATP acted from outside and was dephosphorylated by an ecto-ATPase of the cells. 2. In addition to its direct action, ATP enhanced succinate-dependent calcium uptake in a cooperative fashion. This is best explained by different sites of action. ATP increases cell membrane permeability while succinate stimulates uptake into mitochondria. 3. ATP was able to lower Na+ and K+ gradients and the pH gradient between cells and incubation medium. Increasing calcium concentration counteracted this effect though calcium uptake was then stimulated. 4. Succinate alone did not affect monovalent cation gradients but raised the pH gradient. It partially counteracted the ATP effects on these gradients. 5. Since catecholamine-like actions of ATP may be mediated by an increase in cytoplasmic calcium concentration, the action of extracellular ATP can be taken as a model to study the role of calcium as a transmitter of hormone actions. From interdependence between ATP-stimulated and succinate-stimulated calcium uptake, conclusions can be drawn on the resulting cytoplasmic calcium concentration and its effect on plasma membrane permeability.

Ecto-protein kinase activity in rabbit peritoneal polymorphonuclear leucocytes

An ectoprotein kinase activity has been identified on intact rabbit peritoneal polymorphonuclear leucocytes and the time course of phosphate incorporation into proteins has been followed at different ATP levels. Saturation is reached at around 3 mM ATP and the activity is inhibited by p-chloromercuribenzoate. The possibility that the observed protein phosphorylation arises through the action of a membrane ATPase liberating phosphate for transfer into the cell, incorporation into ATP and its utilisation by endogenous kinases, has been excluded by studying both enzymes concomitantly and measuring the rate of [32P]orthophosphate uptake. Lactate dehydrogenase measurements in the extracellular media also exclude the possibility of kinase liberation from lysed cells. Moreover, the pattern of 32P-labelling of polypeptides when intact cells are exposed to [32P]ATP is quite different from that when homogenates are incubated with [32P]ATP or intact cells with [32P]-orthophosphate. We have been unable to demonstrate any cAMP dependency for this ectokinase activity.

Ecto-ATPase

An ecto-adenosine triphosphatase (E.C. 3.6.1.4 ATP-phosphohydrolase) is shown to be localized on the outer surface of varieties of cell membrane. The enzyme is different from the ATPase involved in biological energy transduction and ion transport mechanism. The characteristic of the enzyme lies in having a very broad substrate specificity and is inhibited by EDTA and higher concentration of ATP. The enzyme is dependent on bivalent metal ions, Mg++ or Ca++ for its optimum activity. The enzyme is highly sensitive to SH-reagents but insensitive to inhibitors of mitochondrial ATPase or Na+- K+- ATPase. The possible functions of the enzyme in being oriented outside the cell membrane is discussed.

Enzymic characteristics of an ecto-cyclic AMP-dependent protein kinase in rat epididymal spermatozoa

Intact spermatozoa from rat cauda epididymides possess an ecto-(cyclic AMP-dependent protein kinase) activity that causes the transfer of the terminal phosphate group of ATP to the serine residues of all the histone fractions. The enzyme showed a high degree of substrate specificity for the phosphorylation of histones rather than protamine, casein and phosvitin. The cell-external-surface protein kinase requires Mg2+ for activity, and other bivalent cations such as Mn2+ and Co2+ can substitute partially for Mg2+, whereas Ca2+ and Zn2+ are potent inhibitors of the enzyme. The enzyme has markedly higher affinity for cyclic AMP than for other cyclic nucleotides for its activation, with an apparent Km value for cyclic AmP of 80 nM. Spermatozoal ecto-kinase activity is not due to contamination of broken cells or any possible cell damage during incubation and isolation of spermatozoa. There was no loss of kinase activity from the cells when washed with 2 mM-EDTA, and the histones phosphorylated by intact spermatozoa were located outside the cells. Protein kinase activity of intact cells was strongly inhibited (approx. 90%) by p-chloromercuribenzenesulphonic acid (10 microM), which is believed not to enter the cells. These data provide further support for the localization of a protein kinase on the external surface of spermatozoa.

Dog red blood cells: Na and K diffusion potentials with extracellular ATP

External ATP causes a prompt increase in the Na and K permeability of dog red blood cells. By manipulating intra- and extracellular ion composition it is possible to observe ATP-induced net fluxes which can be explained in terms of the contribution of Na or K diffusion potentials to the membrane potential. Measurements of membrane voltage by a fluorescent dye technique confirm the existence of such potentials. A rough calculation of chloride permeability gives a value of the order of 10(-8) cm/s, which agrees with results in other species. The cells appear to be somewhat more permeable to bromide than to chloride.

Methods for rapidly altering the permeability of mammalian cells

Various agents alter mammalian cells so that they rapidly become nonspecifically permeable to substances that ordinarily do not penetrate intact cells. Thus, toluene renders liver cells permeable to nucleotides and macromolecules. Tween 80 and Tween 60 act on similar fashion, and the effect is reversible. Dextran sulfate reversibly alters the permeability of Ehrlich ascites tumor cells, which offers a tool for studying the control of macromolecular syntheses and other processes. Brief exposure to external ATP alters the permeability of certain transformed mouse cells but not of untransformed cells. The effect of ATP is rapidly reversible.

Mg2+- and Ca2+-stimulated ATPase at the outer surface of human peripheral lymphocytes and hematopoietic cell lines. Correlation between enzyme activity and immunoglobulin secretion

A Mg2+- and Ca2+-dependent ATPase at the outer surface of human peripheral lymphocytes and hematopoietic cell lines has been studied. The enzyme activity varied about 60 fold between the extremes of the cell lines tested. There was no simple relationship between surface-ATPase activity and cell surface area. Nor was the presence of villi or any other morphological characteristic at the cell surface decisive for the degree of surface-ATPase activity. A correlation was, however, found between the ATPase activity and a phenomenon probably involving a contractile process namely immunoglobulin secretion, but it was not possible to establish an interdependence of these cell characteristics.