Transphosphorylation and G protein activation

Expression of the tumor metastatic suppressor gene in mouse melanoma model: inverse association to metastatic potential

Data obtained in experimental cutaneous melanomas have suggested that the nm23 gene may function as a metastasis suppressor gene. The nm23 level in 8 human cutaneous melanoma cell lines and 2 murine melanoma cell lines were examined. Each melanoma cell line was transplanted subcutaneously into the flank of nude mice, and the metastatic behavior was evaluated by counting lung tumor foci and by determining host survival time. It was found that expression of nm23 mRNA in human melanomas is correlated closely with reduced metastatic behavior in experimental animals and may serve as a sensitive prognostic indicator of malignancy and survival in patients with melanomas.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

Functionally nonequivalent interactions of guanosine 5'-triphosphate, inosine 5'-triphosphate, and xanthosine 5'-triphosphate with the retinal G-protein, transducin, and with Gi-proteins in HL-60 leukemia cell membranes

G-proteins mediate signal transfer from receptors to effector systems. In their guanosine 5'-triphosphate (GTP)-bound form, G-protein alpha-subunits activate effector systems. Termination of G-protein activation is achieved by the high-affinity GTPase [E.C. 3.6.1.-] of their alpha-subunits. Like GTP, inosine 5'-triphosphate (ITP) and xanthosine 5'-triphosphate (XTP) can support effector system activation. We studied the interactions of GTP, ITP, and XTP with the retinal G-protein, transducin (TD), and with G-proteins in HL-60 leukemia cell membranes. TD hydrolyzed nucleoside 5'-triphosphates (NTPs) in the order of efficacy GTP > ITP > XTP. NTPs eluted TD from rod outer segment disk membranes in the same order of efficacy. ITP and XTP competitively inhibited TD-catalyzed GTP hydrolysis. In HL-60 membranes, the chemoattractants N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) and leukotriene B4 (LTB4) effectively activated GTP and ITP hydrolysis by Gi-proteins. fMLP and LTB4 were at least 10-fold more potent activators of ITPase than of GTPase. Complement C5a effectively activated the GTPase of Gi-proteins but was only a weak stimulator of ITPase. The potency of C5a to activate GTP and ITP hydrolysis was similar. The fMLP-stimulated GTPase had a lower Km value than the fMLP-stimulated ITPase, whereas the opposite was true for the Vmax values. fMLP, C5a, and LTB4 did not stimulate XTP hydrolysis. Collectively, our data show that GTP, ITP, and XTP bind to G-proteins with different affinities, that G-proteins hydrolyze NTPs with different efficacies, and that chemoattractants stimulate GTP and ITP hydrolysis by Gi-proteins in a receptor-specific manner. On the basis of our results and the data in the literature, we put forward the hypothesis that GTP, ITP, and XTP act as differential signal amplifiers and signal sorters at the G-protein level.

Effects of the wasp venom peptide, mastoparan, on GTP hydrolysis in rat brain membranes

1. The effects of mastoparan, a wasp venom toxin, on GTP hydrolyzing activity were examined in rat brain membranes. 2. Mastoparan inhibited the low-affinity GTPase activity, defined as the amount of 32Pi released from 0.3 microm [gamma-32P]-GTP in the presence of 100 microM unlabelled GTP, in a concentration-dependent manner. This inhibitory effect of mastoparan on low-affinity GTPase activity was diminished by increasing concentrations of UDP and was completely attenuated at 20 mM, indicating that activation of nucleoside diphosphokinase (NDPK) is inolved in the phenomenon. 3. In the presence of 20 mM UDP, mastoparan stimulated the high-affinity GTPase activity by increasing the Vmax value without affecting the apparent K(M) for GTP. Mastoparan-stimulated high-affinity GTPase activity was apparent at concentrations higher than 1 microM, in a concentration-dependent manner, but without saturation even at 100 microM. 4. Mastoparan-induced high-affinity GTPase activity showed a characteristic sensitivity to MgCl2, quite different from that seen in L-glutamate-stimulated activity, a representative of receptor-mediated G-protein activation. 5. There appeared to be a simple additive interaction between mastoparan- and L-glutamate-stimulated high-affinity GTPase activities, indicting that distinct pools of G-proteins are involved in receptor-independent and receptor-mediated G-protein activation. 6. These results suggest that G-proteins in brain membranes are functionally altered by mastoparan through multiple mechanisms of action and that the mastoparan-induced, direct G-protein activating process lacks a synergistic or antagonistic interaction with an agonist-induced, receptor-mediated activation of G-proteins.

Participation of nucleoside-diphosphate kinase in muscarinic K+ channel activation does not involve GTP formation

Agonist-bound muscarinic receptors open atrial K+ channels through a GTP-dependent pathway mediated by the G protein Gk. However, nucleotides other than GTP are also able to support channel activity, even in the absence of agonists. This process was proposed to be mediated by nucleoside-diphosphate (NDP) kinase, which would transfer phosphate from nucleotide triphosphates to the GDP bound to Gk, producing Gk-GTP without the need for receptor-induced GDP-GTP exchange. We examined the effect of antibodies to NDP kinase on the ATP-supported activity of atrial muscarinic K+ channels and the corresponding GIRK1/CIR channels expressed in HEK 293 cells. Inhibitory antibodies reduced ATP-induced channel openings, but this effect displayed an absolute requirement for agonist and was also seen with antibodies that do not inhibit the enzyme. Both types of antibodies also reduced agonist-dependent channel activity in the presence of GTP, ruling out a role for NDP kinase in GDP rephosphorylation. Channel activity was not affected by the antibodies in preparations where ATP-induced muscarinic channels are not under tight receptor control, namely pertussis toxin-treated atrial patches and membranes from cells expressing KACh channel subunits. Thus, participation of NDP kinase in this pathway requires activated receptors and has a function distinct from phosphate transfer between nucleotides.

Transducin-mediated, isoform-specific interaction of recombinant rat nucleoside diphosphate kinases with bleached bovine retinal rod outer segment membranes

The properties of the binding of recombinant rat nucleoside diphosphate (NDP) kinase isoforms alpha and beta (NDP kinase alpha and beta, respectively) to bleached bovine retinal rod outer segment (ROS) membranes were investigated. It was found that: (1) both NDP kinase isoforms interacted with ROS membranes in a pH-, cation- and GTPgammaS-dependent manner; (2) the retinal G-protein transducin was an obligatory factor for the interaction; (3) the apparent affinity of NDP kinase alpha for ROS membranes was about 100-fold higher than that of NDP kinase beta; and (4) an alpha-isoform-specific peptide, corresponding to the sequence of the N-terminal third (variable region), had the ability to displace bovine NDP kinase from ROS membranes. The results suggest the possible involvement of NDP kinases in cellular regulation via interaction with G-proteins and provide a structural basis for the possible differential roles of mammalian NDP kinase isoforms in the cell.

Muscarinic acetylcholine receptor trafficking in streptolysin O-permeabilized MDCK cells

We investigated the validity of streptolysin O (SLO)-permeabilized Madin-Darbin canine kidney (MDCK) cells which express muscarinic acetylcholine receptors (mAChRs) coupled to pertussis toxin-sensitive guanine nucleotide-binding proteins (G proteins) for the study of the molecular machinery that regulated mAChR internalization and recycling. Exposure of SLO-permeabilized cells to carbachol-reduced cell surface receptor number by up to 40% without changing total receptor number. The kinetics and maximal extent of receptor internalization as well as the potency of carbachol to induce receptor internalization were almost identical in SLO-permeabilized and non-permeabilized cells. Using this semi-intact cell system, we studied the effect of various agents affecting components potentially involved in receptor trafficking. Internalization was prevented by treatment of the SLO-permeabilized MDCK cells with (i) the stable ATP analogues, adenosine 5'-O-(3-thiotriphosphate) and adenylylimidodiphosphate, to block ATP-dependent processes, and (ii) heparin to block G protein-coupled receptor kinases. Inclusion of the stable GTP analogue, guanosine 5'-O-(3-thiotriphosphate), increased the rate but not the extent of receptor internalization. None of the membrane-impermeant agents affected receptor internalization in intact MDCK cells. This model system also allowed recycling of internalized receptors back to the plasma membrane. After removal of the agonist, cell surface receptor number in SLO-permeabilized cells returned to control values within 90 min with the same kinetics as seen in intact cells. Inclusion of guanosine 5'O-(3-thiotriphosphate) shortened the recovery time. These data suggest that both ATP-dependent kinases including G protein-coupled receptor kinases and G proteins participate in receptor internalization and recycling. In summary, the SLO-permeabilized MDCK cell is a feasible model system for the study of mAChR internalization and recycling and allows manipulation of the intracellular milieu with membrane-impermeable macromolecules.

Phosphotransfer reactions as a means of G protein activation

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) serve to transduce information from agonist-bound receptors to effector enzymes or ion channels. Current models of G protein activation-deactivation indicate that the oligomeric GDP-bound form must undergo release of GDP, bind GTP and undergo subunit dissociation, in order to be in active form (GTP bound alpha subunits and free beta gamma dimers) and to regulate effectors. The effect of receptor occupation by an agonist is generally accepted to be promotion of guanine nucleotide exchange thus allowing activation of the G protein. Recent studies indicate that transphosphorylation leading to the formation of GTP from GDP and ATP in the close vicinity, or even at the G protein, catalysed by membrane-associated nucleoside diphosphate kinase, may further activate G proteins. This activation is demonstrated by a decreased affinity of G protein-coupled receptors for agonists and an increased response of G protein coupled effectors. In addition, a phosphorylation of G protein beta subunits and consequent phosphate transfer reaction resulting in G protein activation has also been demonstrated. Finally, endogenously formed GTP was preferentially effective in activating some G proteins compared to exogenous GTP. The aim of this report is to present an overview of the evidence to date for a transphosphorylation as a means of G protein activation (see also refs [1 and 2] for reviews).

Neutralizing antibodies to nucleoside diphosphate kinase inhibit the enzyme in vitro and in vivo: evidence for two distinct mechanisms of activation of atrial currents by ATPgammaS

Nucleoside diphosphate kinase (NDPK) participates in multiple cellular functions, yet the molecular mechanisms of its involvement are often unknown, given that there are no specific inhibitors for the enzyme from vertebrates. We developed antibodies against NDPK by immunization of rabbits with the enzyme from bullfrog skeletal muscle. The antibodies specifically recognized the enzyme from frog tissues, and cross-reacted with NDPK from Xenopus. In contrast to mammalian NDPK, the amphibian enzyme elicited antibodies that inhibit potently its catalytic function. We utilized the inhibitory properties of these immunoglobulins to examine the role of NDPK on the ATPgammaS-induced stimulation of Ca2+ and K+ currents of cardiac myocytes. Injection of NDPK-neutralizing Fab fragments into atrial cells reduced considerably the effect of ATPgammaS on muscarinic K+ currents, but not on Ca2+ currents. Therefore, ATPgammaS increases calcium and potassium currents of atrial cells by two distinct mechanisms. NDPK is essential for the conversion of ATPgammaS into GTPgammaS which leads to muscarinic K+ channel activation but not for the stimulation of Ca2+ currents by ATPgammaS. The results demonstrate that antibodies to frog NDPK block the activity of the enzyme in vivo and in vitro, and can be used to determine the relevance of NDPK and its catalytic activity to the function of vertebrate cells.

Activation of GTP formation and high-affinity GTP hydrolysis by mastoparan in various cell membranes. G-protein activation via nucleoside diphosphate kinase, a possible general mechanism of mastoparan action

The wasp venom, mastoparan (MP), is a direct activator of reconstituted pertussis toxin-sensitive G-proteins and of purified nucleoside diphosphate kinase (NDPK) [E.C. 2.6.4.6.]. In HL-60 membranes, MP activates high-affinity GTPase [E.C. 3.6.1.-] and NDPK-catalyzed GTP formation, but not photolabeling of G-protein alpha-subunits with GTP azidoanilide; this suggests that the venom activates G-proteins in this system indirectly via stimulation of NDPK. Moreover, the MP analogue, mastoparan 7 (MP 7), is a much more effective activator of reconstituted G-proteins than MP, whereas with regard to NDPK and GTPase in HL-60 membranes, the two peptides are similarly effective. In our present study, we investigated NDPK- and G-protein activation by MP in membranes of the human neuroblastoma cell line, SH-SY5Y, the human erythroleukemia cell line, HEL, the rat basophilic leukemia cell line, RBL 2H3, and the hamster ductus deferens smooth muscle cell line, DDT1MF-2. All these membranes exhibited high NDPK activities that were increased by MP. Compared to basal GTP formation rates, basal rates of high-affinity GTP hydrolysis in cell membranes were low. MP activated high-affinity GTP hydrolysis in cell membranes but did not enhance incorporation of GTP azidoanilide into G-protein alpha-subunits. As with HL-60 membranes, MP and MP 7 were similarly effective activators of NDPK and GTPase in SH-SY5Y membranes. Pertussis toxin inhibited MP-stimulated GTP hydrolyses in SH-SY5Y- and HEL membranes, whereas NDPK activations by MP were pertussis toxin-insensitive. Our data suggest that indirect G-protein activation via NDPK is not restricted to HL-60 membranes but is a more general mechanism of MP action in cell membranes. Pertussis toxin-catalyzed ADP-ribosylation of alpha-subunits may inhibit the transfer of GTP from NDPK to G-proteins. NDPK may play a much more important role in transmembrane signal transduction than was previously appreciated and, moreover, the GTPase of G-protein alpha-subunits may serve as GDP-synthase for NDPK.

Purification of chlorpromazine-sensitive GTPase from rat cerebral cortex

The chlorpromazine-sensitive GTPase from the cell membrane of rat cerebral cortex was purified to homogenity by using DEAE Bio-Gel A agarose, hydroxyapatite and heparin agarose chromatography. The purified chlorpromazine-sensitive GTPase was purified 370-fold to obtain a final specific activity of 40 mumol GTP hydrolyzed2min/mg protein. The purified enzyme was inhibited by chlorpromazine but not by compound 48/80. Magnesium was required for its activity instead of calcium. The purified enzyme had an apparent pH optimum of 8.0, and molecular weight was estimated to be 58,000.

Direct and indirect receptor-independent G-protein activation by cationic-amphiphilic substances. Studies with mast cells, HL-60 human leukemic cells and purified G-proteins

Studies from several laboratories have revealed that structurally diverse substances including the wasp venom, mastoparan (MP), activate purified regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, presumably by mimicking the effects of heptahelical receptors. Mast cells and differentiated HL-60 human leukemic cells are useful model systems for the analysis of receptor-independent G-protein activation. We compared the effects of 2-phenylhistamines which are cationic-amphiphilic, too, and of MP on G-protein activation in dibutyryl cAMP-differentiated HL-60 cells and in the rat basophilic leukemia cell line, RBL 2H3. In HL-60 cells, 2-phenylhistamines show stimulatory effects which resemble those of formyl peptide receptor agonists but which cannot be attributed to agonism at classical receptors. 2-phenylhistamines do not, however, activate RBL 2H3 cells and various other myeloid cell types, pointing to cell type-specificity of receptor-independent G-protein activation. In HL-60 cells, MP shows effects on G-protein activation which differ substantially from those of formyl peptides. In RBL 2H3 membranes, MP shows similar effects on G-protein activation as in HL-60 membranes. We develop a model according to which receptor-independent G-protein activation can be subdivided into direct and indirect receptor-independent G-protein activation. In case of the former mechanism, substances like 2-phenylhistamines interact with G-protein alpha-subunits and in case of the latter mechanism, substances like MP interact with nucleoside diphosphate kinase which catalyzes the formation of GTP. This newly formed GTP is then transferred to, and cleaved by, G-protein alpha-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by cromoglycate and some flavonoids of nucleoside diphosphate kinase and of exocytosis from permeabilized mast cells

1. The anti-allergic compound, cromoglycate, is reported to possess affinity for, and to suppress the autophosphorylation of a 72kDa protein having the sequence of nucleoside diphosphate kinase (NDPK). 2. We have tested the ability of cromoglycate, and a panel of ten structurally related flavonoids of plant origin, to inhibit the NDPK reaction and the exocytotic process of permeabilized mast cells. The conditions of permeabilization (use of an isotonic medium based on sodium glutamate) were selected to ensure that NDPK activity would be an essential component in the induction of Ca(2+)-induced exocytosis in which ATP is required for generation of GTP. For comparison, we also measured the inhibition of exocytosis induced by GTP-gamma-S; this proceeds in the absence of ATP and bypasses the need for NDPK activity. 3. We found that cromoglycate does not discriminate between Ca2+ and GTP-gamma-S-induced exocytosis and is a poor inhibitor of NDPK activity. Concentrations in the millimolar range are required for inhibition of all these functions. By comparison, many of the flavonoids are effective at concentrations in the micromolar range. 4. While we were unable to discern any systematic relationships between their ability to inhibit the three functions, two compounds, quercetin and genistein, inhibit Ca(2+)-induced, but not GTP-gamma-S-induced exocytosis. Inhibition of the late stages of the stimulus-response pathway in mast cells by these compounds is therefore likely to be due to inhibition of NDPK and the consequent failure to generate GTP.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

Cytosolic biosynthesis of GTP and ATP in normal rat pancreatic islets

GTP and ATP are necessary for glucose-induced insulin secretion; however, the biosynthetic pathways of purine nucleotides have not been studied in pancreatic islets. The present work examines the cytosolic pathways of purine nucleotide synthesis using intact rat islets cultured overnight in RPMI 1640 medium containing either [14C]glycine (to label the de novo pathway) or [3H]hypoxanthine (to mark the salvage pathway), with or without mycophenolic acid or L-alanosine (selective inhibitors of cytosolic GTP and ATP synthesis, respectively). Addition of mycophenolic acid decreased total GTP content (mass) by 73-81%; although the incorporation of labeled hypoxanthine into GTP also fell by 87%, the incorporation of glycine did not change. Similarly, L-alanosine decreased ATP mass by 26-33% in the presence of either label; whereas the incorporation of hypoxanthine into ATP fell 59%, the incorporation of glycine was again not significantly decreased. Thus, both the de novo and salvage purine nucleotide biosynthetic pathways are present in rat islets; however, the salvage pathway appears to be quantitatively the more important source of nucleotides. This conclusion was supported by additional studies of the effects on nucleotide content and insulin secretion of various site-specific inhibitors of purine synthesis. These findings have potential relevance to the processes of mitogenesis, cell proliferation and differentiation of islet cells, as well as for the control of insulin secretion.

gamma-Aminobutyric acidA receptor function is modulated by cyclic GMP

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate nervous system. Modulatory effects of intracellular ATP on the GABA response in isolated bullfrog dorsal root ganglion neurons were examined using whole-cell voltage clamp. Investigation of the plausible mechanisms ATP might utilize to regulate the GABA response led to the discovery that intracellular cyclic GMP may play an important role in modulating inhibitory neurotransmission. This modulatory effect of cyclic GMP is likely to be mediated via a cyclic GMP-dependent protein kinase.

Mastoparan may activate GTP hydrolysis by Gi-proteins in HL-60 membranes indirectly through interaction with nucleoside diphosphate kinase

The wasp venom, mastoparan (MP), activates reconstituted pertussis toxin (PTX)-sensitive G-proteins in a receptor-independent manner. We studied the effects of MP and its analogue, mastoparan 7 (MP 7), on G-protein activation in HL-60 cells and a reconstituted system and on nucleoside diphosphate kinase (NDPK)-catalysed GTP formation. MP activated high-affinity GTP hydrolysis in HL-60 membranes with an EC50 of 1-2 microM and a maximum at 10 microM. Unlike the effects of the formyl peptide receptor agonist, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe), on GTPase, those of MP were only partially PTX-sensitive. MP-induced rises in cytosolic Ca2+ concentration and superoxide-anion formation in intact HL-60 cells were also only incompletely PTX-sensitive. N-Ethylmaleimide inhibited MP-stimulated GTP hydrolysis to a greater extent than that stimulated by fMet-Leu-Phe. Unlike the latter, MP did not enhance incorporation of GTP azidoanilide into, and cholera toxin-catalysed ADP-ribosylation of, Gi-protein alpha-subunits in HL-60 membranes. By contrast to fMet-Leu-Phe, MP did not or only weakly stimulated binding of guanosine 5'-[gamma-thio]triphosphate to Gi-protein alpha-subunits. MP 7 was considerably more effective than MP at activating the GTPase of reconstituted Gi/G(o)-proteins, whereas in HL-60 membranes, MP and MP 7 were similarly effective. MP and MP 7 were similarly effective at activating [3H]GTP formation from [3H]GDP and GTP in HL-60 membranes and by NDPK purified from bovine liver mitochondria. Our data suggest the following: (1) MP activates Gi-proteins in HL-60 cells, but (2) the venom does not simply mimic receptor activation. (3) MP and MP 7 may activate GTP hydrolysis in HL-60 membranes indirectly through interaction with NDPK. (4) MP 7 is a more effective direct activator of PTX-sensitive G-proteins than MP, whereas with regard to NDPK, MP and MP 7 are similarly effective.

Differential nm23 gene expression at the fetal-maternal interface

The product of the nm23 gene has been proposed as a candidate tumour metastasis suppressor protein. A strong association has been observed between reduced expression of the nm23 gene and acquisition of metastatic behaviour in some tumour cells, including breast cancer and melanoma, but not in others, such as neuroblastoma and colon, cervical and thyroid cancers. During the early gestation period both human and murine trophoblast cells exhibit in vitro invasive properties similar to those of neoplastic cells. Such invasive properties, however, disappear in the late stage of gestation. In the present study, we examined the abundance of nm23 mRNA from various fetal-maternal interface tissues (uterus, decidua, placenta and embryo) during early (day 8), mid (day 14) and late (day 18) stages of gestation in CD1 mice, in order to determine whether nm23 plays any anti-invasive and/or biological roles during gestation. nm23 was found to be expressed in all the tissues during the early and mid stages of gestation. The expression levels were, however, variable among different tissues and development stages. In the early stage, nm23 mRNA levels were the highest and similar among tissues from the uterus, decidua, placenta and embryo. In the mid stage, the mRNA levels were reduced significantly in the uterus, decidua and placenta, but not in the embryo. In the late stage, nm23 mRNA was further reduced to the extent that it could not be seen in the decidua, was barely seen in the uterus and was weakly present in the placenta. However, the mRNA level of the embryo in the late stage was still high and similar to the early stage. We also examined nm23 expression in trophoblast cells from normal human term placenta and a highly metastatic human choriocarcinoma cell line, JAR. nm23 expression was significantly higher in JAR than in normal placenta, indicating that nm23 does not appear to have an anti-metastatic function in this cell line. Several cytokines--interleukin 2 (IL-2), tumour necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma)--and prostaglandin E2 (PGE2) known to modulate tumour growth and metastasis were examined to determine whether they regulate nm23 expression in JAR in vitro. The B16F10 melanoma cell line was used as control. No effect was found in the JAR cell line, whereas TNF-alpha, IFN-gamma and PGE2 down-regulated nm23 expression in the B16F10 cell line.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of phospholipase D in rabbit platelet membranes by nucleoside triphosphates and by phosphocreatine: roles of membrane-bound GDP, nucleoside diphosphate kinase and creatine kinase

Previous work has shown that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and GTP stimulate phospholipase D (PLD) in rabbit platelet membranes and that these effects are greatly enhanced by pretreatment of platelets with phorbol esters that activate protein kinase C [Van der Meulen and Haslam (1990), Biochem. J. 271, 693-700]. In the present study, the effects of Mg2+, various nucleoside triphosphates and phosphocreatine (PCr) were investigated. Platelet membranes containing phospholipids labelled with [3H]glycerol were assayed for PLD in the presence of an optimal Mg2+ concentration (10 mM) by measuring [3H]phosphatidylethanol formation in incubations that included 300 mM ethanol. In membranes from phorbolester-treated platelets, the same maximal increases in PLD activity (5-fold) were seen with 1 microM GTP[S]), and 100 microM GTP. Addition of adenosine 5'-[gamma-thio]triphosphate (ATP[S]), ITP, XTP, UTP and CTP had similar stimulatory effects, but only at > or = 1 mM. In contrast, ATP had a biphasic action, causing a maximal (2-fold) stimulation at 10 microM and smaller effects at higher concentrations; the inhibitory component of the action of ATP was blocked by 2 microM staurosporine. Guanosine 5'-[beta-thio]diphosphate decreased the stimulatory effects of ATP and ATP[S]. UDP, which can inhibit nucleoside diphosphate kinase (NDPK), decreased the activation of PLD by ATP[S], ATP, XTP, CTP and to a lesser extent ITP, but had no effect on the actions of GTP[S] and GTP. Rabbit platelet membranes contained NDPK and addition of [gamma-32P]ATP led to the formation of [32P]GTP in amounts sufficient to explain most or all of the activation of PLD; UDP prevented GTP formation. PCr (0.04-1 mM) also stimulated membrane PLD activity, an effect that was dependent on endogenous membrane-bound creatine kinase (CK). UDP and guanosine 5'-[beta-thio]diphosphate each inhibited this effect of PCr. The results show that in rabbit platelet membranes, CK, NDPK and the GTP-binding protein that activates PLD can be functionally coupled. However, assay of membrane preparations at increasing dilutions showed that stimulation of PLD by the compounds studied, with the partial exception of ATP[S], involved diffusible rather than protein-bound intermediates.

Recombinant rat nucleoside diphosphate kinase isoforms (alpha and beta): purification, properties and application to immunological detection of native isoforms in rat tissues

We previously demonstrated that at least two isoforms of nucleoside diphosphate (NDP) kinase, the products of two different tandemly arrayed genes, are present in rat. To understand the physiological role of each isoform, some biochemical properties of recombinant rat NDP kinase alpha- and beta-isoforms, produced in large amount, were studied. cDNAs of the two isoforms were inserted in an expression vector pET3b and recombinant enzymes were overproduced in Escherichia coli. Their primary structures were different from the native enzymes in that the latter suffer from modification of the NH2-terminal end. The two recombinant isoforms were purified from the cell lysate to apparent homogeneity by ammonium sulfate fractionation followed by three successive column chromatographies. Despite their extreme similarity in the amino-acid sequences, the two showed somewhat different enzymic properties in terms of di- and triphosphate nucleotide substrate specificity. They showed similar mobilities on SDS-PAGE as expected from their calculated molecular weight (alpha-isoform, 17,283 versus beta-isoform, 17,192) but differed in isoelectric point (alpha-isoform, pI 6.7; beta-isoform, pI 7.8) and heat stability. Polyclonal antibody which reacted with both isoforms and alpha-isoform-specific monoclonal antibodies differentially recognized native enzymes from rat tissues after the tissue extracts were separated by isoelectric focusing gel electrophoresis under a denaturation condition. The results showed that the alpha-isoform, though its amount varied from one tissue to another, was the major form in rat tissues examined compared with the beta-isoform which was detectable in brain and testis. There was no preference in their subcellular localization when examined with myelin, synaptosomal supernatant and total homogenate fractions from the rat cerebrum and cerebellum.

Clinicopathological features and prognostic significance of nucleoside diphosphate kinase/nm23 gene product in human pancreatic exocrine neoplasms

Expression levels of nucleoside diphosphate (NDP) kinase/nm23 gene product in the surgically resected 59 human pancreatic exocrine neoplasms were examined immunohistochemically using antiNDP kinase antibody. Immunoreactivity for NDP kinase varied from one tumor to the other. Out of the 47 invasive pancreatic duct cell carcinomas examined, 31 (66%) tumors showed strong immunoreactivity for NDP kinase, whereas ten (83%) of 12 benign or less invasive tumors (in situ adenocarcinoma and mucin-producing tumor) showed negative or weak immunoreactivity (p < 0.01; Chi-square test). Overall survival of invasive pancreatic duct cell carcinomas with strong immunoreactivity for NDP kinase was poorer than those with negative or weak immunoreactivity (p < 0.03; generalized Wilcoxon test). Strong immunoreactivity for NDP kinase was associated with the type of histological differentiation, the presence of lymph node metastases (p < 0.05, respectively; Chi-square test), and the number of argyrophilic nucleolar organizer regions (p < 0.01; Student's t-test). These results suggest that NDP kinase/nm23 gene product expression was positively associated with tumor aggressiveness and poor survival of patients in human pancreatic exocrine neoplasms. They also suggest that NDP kinase expression is related to cell proliferation activity represented by the number of argyrophilic nucleolar organizer regions. Therefore, examining the level of NDP kinase/nm23 gene product could serve as a marker for malignant potentiality of pancreatic exocrine neoplasms.

On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells

1. Acetylcholine (ACh) produces two membrane current changes when applied to NG108-15 mouse neuroblastoma x rat glioma hybrid cells transformed (by DNA transfection) to express m1 muscarinic receptors: it activates a Ca(2+)-dependent K+ conductance, producing an outward current, and it inhibits a voltage-dependent K+ conductance (the M conductance), thus diminishing the M-type voltage-dependent K+ current (IK(M)) and producing an inward current. The present experiments were undertaken to find out how far inhibition of IK(M) might be secondary to stimulation of phospholipase C, by recording membrane currents and intracellular Ca2+ changes with indo-1 using whole-cell patch-clamp methods. 2. Bath application of 100 microM ACh reversibly inhibited IK(M) by 47.3 +/- 3.2% (n = 23). Following pressure-application of 1 mM ACh, the mean latency to inhibition was 420 ms at 35 degrees C and 1.79 s at 23 degrees C. Latencies to inhibition by Ba2+ ions were 148 ms at 35 degrees C and 92 ms at 23 degrees C. 3. The involvement of a G-protein was tested by adding 0.5 mM GTP-gamma-S or 10 mM potassium fluoride to the pipette solution. These slowly reduced IK(M), with half-times of about 30 and 20 min respectively, and rendered the effect of superimposed ACh irreversible. Effects of ACh were not significantly changed after pretreatment for 24 h with 500 ng ml-1 pertussis toxin or on adding up to 10 mM GDP-beta-S to the pipette solution. 4. The role of phospholipase C and its products was tested using neomycin (to inhibit phospholipase C), inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4), heparin, and phorbol dibutyrate (PDBu) and staurosporin (to activate and inhibit protein kinase C respectively). Both neomycin (1 mM external) and InsP3 (100 microM intrapipette) inhibited the ACh-induced outward current and/or intracellular Ca2+ transient but did not block ACh-induced inhibition of IK(M). Intrapipette heparin (1 mM) blocked activation of IK(Ca) and reduced Ach-induced inhibitions of IK(M), but also reduced inhibition of ICa via endogeneous m4 receptors. PDBu (with or without intrapipette ATP) and staurosporin had no significant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

High levels of Nm23 gene expression in advanced stage of thyroid carcinomas

The product of Nm23 gene has been proposed as a candidate tumour metastasis suppressor protein. A strong association has been observed between reduced expression of Nm23 gene and acquisition of metastatic behaviour in some tumour cells including breast cancer and melanoma, but not in others such as colon cancer, neuroblastoma, and cervical cancer. In the present study, we examined the abundance of Nm23 mRNA in 39 thyroid tissue specimens including five multinodular goitres, one follicular adenoma, 26 papillary and three follicular carcinomas, and four anaplastic carcinomas. Nm23 was found to be expressed in all the tissue specimens. The expression was, however, variable in different stages of thyroid carcinoma. In stages I through III of differentiated thyroid carcinoma, the average level of Nm23 gene expression was comparable to that in multinodular goitres. In advanced stage of thyroid carcinoma (stage IV and anaplastic), 2-fold increase of Nm23 expression was noted. No mutations were found in the coding region of the gene. Nm23 mRNA level cannot, therefore, be used as a marker of low metastatic potential in thyroid carcinomas. The association of high level Nm23 expression with anaplastic thyroid carcinoma suggests its correlation with rapid cell proliferation.

On the role of G protein activation and phosphorylation in desensitization to acetylcholine in guinea-pig atrial cells

1. The ACh-activated K+ current (IK,ACh) has been investigated in guinea-pig atrial cells at 36 degrees C using the whole-cell patch-clamp technique. 2. During an exposure to ACh, IK,ACh faded as a result of desensitization. Throughout the fade of the current, the current reversed at EK and showed inward-going rectification. The fade was, therefore, the result of a genuine decrease in IK,ACh. 3. The onset of desensitization (as judged by the fade of IK,ACh) was biphasic and the time constants of the fast and slow phases of desensitization were 1.58 +/- 0.14 (n = 16) and 148.2 +/- 12.8 s (n = 18) respectively. Recovery from the fast and slow phases of desensitization (after 30 s and 5 min exposures to ACh respectively) occurred with time constants of 52 and 222 s respectively. This suggests that two processes are involved in desensitization. 4. The Q10 of the rate constant of the fast phase of desensitization was 2.2 +/- 0.3 (n = 6). 5. Intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or extracellular perfusion with AlF4- were used to bypass the muscarinic receptor and trigger IK,ACh by directly activating the G protein, GK, that links the muscarinic receptor to the K+ channel. Both GTP gamma S and AlF4- activated a current with the same reversal potential and the same degree of inward-going rectification as the ACh-activated current. 6. Desensitization still occurred when the muscarinic receptor was bypassed and IK,ACh was triggered by direct activation of GK with either GTP gamma S or AlF4-. This suggests that desensitization is, in part, the result of a modification of either GK or the K+ channel. 7. Activation of the muscarinic receptor by ACh resulted in greater desensitization than direct activation of GK; at the end of a 5 min exposure to ACh, current was only 22 +/- 1% (n = 19) of its peak value, whereas, after direct activation of GK by GTP gamma S for 5 min, current was 42 +/- 6% (n = 5) of its peak value. This suggests that desensitization also involves the muscarinic receptor. 8. When cells were perfused with GTP gamma S, the fast phase of desensitization could still occur, but the slow phase was reduced. This suggests that the fast phase involves GK or the K+ channel, whereas the slow phase involves the muscarinic receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-induced secretion from permeabilized rat mast cells: requirements for guanine nucleotides

Cells used in this work were permeabilized by streptolysin-O and then washed to remove freely soluble components. The secretory responsiveness of these cells to various combinations of calcium, MgATP and guanine nucleotide was characterized and in most respects was found to be similar to that of the metabolically inhibited (unwashed) cells. The content of adenosine and guanine nucleotides, which remain within the permeabilized cells after washing, was estimated as 0.83 and 0.12 mM (extrapolated to intact cells), which constitutes 18 and 25%, respectively, of the total nucleotide content of mast cells. High (> mM) concentrations of MgATP, required for the calcium-induced secretion, were reduced to microM levels by suboptimal concentrations of GTP, which also markedly increased both the rate and extent of the response. Similarly, microM concentrations of MgATP reduced the requirements of the calcium-dependent secretion for GTP. The synergy of the GTP and ATP effects suggests that, together, the two nucleotides can maintain a pool of free GTP, presumably as a result of transphosphorylation from ATP to GDP. Thus, MgATP may work by transphosphorylating the endogenous GDP. However, neither GTP nor GTP-gamma-S were effective as substitutes for MgATP in the calcium-induced secretion, particularly that from metabolically inhibited cells. This indicates that MgATP does not act simply by providing GTP but is needed to maintain a phosphorylated state of the system. The synergistic effects of ATP and GTP were observed only in the presence of calcium. To test whether calcium/MgATP-induced secretion requires an activated G protein, the effects of G-protein inactivators were studied. GDP, deoxy GDP and GDP-beta-S exerted differing degrees of inhibition on secretory responses induced by various combinations of effectors. The response to calcium/MgATP was less sensitive to these inhibitors than that to GTP-gamma-S (with or without calcium). However, all three 'inhibitors' were also capable of stimulating calcium/MgATP-dependent secretion, indicating a transphosphorylation, producing GTP, dGTP and GTP-beta-S. Thus, in the absence of any specific inhibitors for either G proteins or the transphosphorylation reaction, the degree of dependence of the calcium-induced secretion on a G protein remains unclear.

Expression of nucleoside diphosphate kinase/nm23 gene product in human pancreatic cancer: an association with lymph node metastasis and tumor invasion

The expression of nucleoside diphosphate (NDP) kinase/nm23 has been reported to be inversely related to metastasizing potential of experimental cells and human breast cancer. In the present study, levels of NDP kinase/nm23 gene product in curatively resected human pancreatic adenocarcinomas were examined immunohistochemically using anti-NDP kinase antibody. Immunoreactivity for NDP kinase varied between tumors. Of 31 pancreatic tumors examined, 17 (55%; positive staining group) showed strong immunoreactivity for the NDP kinase, while 14 (45%; negative staining group) showed low or no immunoreactivity. Positive staining was associated with higher incidence of lymph node metastasis (13/17; 77%) and perineural invasion (13/17; 77%) than negative staining (5/14, 36%, P < 0.03; 4/14, 29%, P < 0.01, respectively). Positive staining was also associated with shorter overall survival and relapse-free survival than negative staining (P < 0.01, P < 0.01, respectively). No significant difference in age, sex, size, location of tumor, serum carcinoembryonic antigen (CEA) level, or histological type was found between the two groups. These results showed that, in contrast to the reports on breast cancer, NDP kinase/nm23 expression in human pancreatic cancer is positively associated with lymph node metastasis or perineural invasion and with poor prognosis. These, together with other previous reports, suggest that NDP kinase may play an important role in cancer progression or aggressiveness by altering its expression in a tissue-specific manner.

Intercellular guanosine-5'-0-(3-thiotriphosphate) blocks chemotactic motility of Dictyostelium discoideum amoebae

Starving amoebae of the cellular slime mold Dictyostelium discoideum react chemotactically towards the attractant cAMP. In this study, the effect of nonhydrolyzable analogs of GTP and GDP on the chemotactic behavior was analyzed with light microscopic techniques. Guanosine-5'-0-(2-thiotriphosphate) (GTP gamma S) or guanosine-5'-0-(2-thiodiphosphate) (GDP beta S) was scrape-loaded into the cytoplasm of cells, together with a fluorescent marker. Stimulation with a cAMP-filled glass capillary revealed a reduced capacity of loaded cells to migrate towards the capillary tip. Most cells still protruded filopods in the direction of the capillary tip, but full extension of pseudopods was inhibited in a dose-dependent and reversible manner. This indicates that in the presence of the analogs, chemotactic sensing still occurs, and that a more distal step of the cascade of events leading to the formation of the pseudopod is impaired. In cells loaded with the analogs together with the calcium indicator fura-2, stimulation with 10 microM cAMP led to a transient change in the intracellular free calcium concentration ([Ca2+]i), which was detectable in 28% of the cells. Furthermore, large vacuoles were found containing high amounts of calcium. On the other hand, clamping of [Ca2+]i at low levels with 1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetraacetic acid (BAPTA) also inhibited motility, with neither filopods nor pseudopods formed. The data suggest that chemotactic migratory activity involves GTP-dependent processes that participate in the regulation of the Ca2+ homeostasis of the cell and in the regulation of membrane traffic that contributes to the directed locomotion.

Acetylcholine-mediated K+ channel activity in guinea-pig atrial cells is supported by nucleoside diphosphate kinase

We studied the role of nucleoside diphosphate kinase (NDPK) in acetylcholine-mediated muscarinic K+ channel activation in inside-out patches of guinea-pig atrial cells. NDPK-catalysed activation of the muscarinic K+ channels by adenosine triphosphate-Mg2+ (ATP-Mg2+) is not prevented by occupation of the muscarinic receptor [by acetylcholine (ACh) or atropine], nor by uncoupling of the receptor from the G protein by pertussis-toxin-catalysed adenosine diphosphate (ADP)-ribosylation of GK. In the presence of ACh, addition of 0.1 mM guanosine triphosphate (GTP) after activation of the channels by 4 mM ATP alone resulted in a moderate increase of channel activity (in contrast to block in the absence of ACh): NDPK-mediated direct transphosphorylation is uncoupled by the G nucleotide but agonist-induced guanosine diphosphate (GDP)-to-GTP exchange takes over activation of the channels. Moreover, ACh-dependent channel stimulation was possible in inside-out patches while ATP and GDP were present in the bathing solution (in contrast to the complete absence of channel activation in the absence of ACh). This indicates that NDPK synthesizes sufficient GTP to support channel activation by exchange. Hence, it is postulated that the main functional role of NDPK under physiological conditions is to provide a local supply of GTP (using GDP and ATP) in the immediate vicinity of the G protein, thereby maintaining a high local GTP/GDP ratio and ensuring adequate receptor-mediated regulation of muscarinic K+ channel activity.

A GTPase controls cell-substrate adhesion in Xenopus XTC fibroblasts

Cell-substrate adhesion is crucial at various stages of development and for the maintenance of normal tissues. Little is known about the regulation of these adhesive interactions. To investigate the role of GTPases in the control of cell morphology and cell-substrate adhesion we have injected guanine nucleotide analogs into Xenopus XTC fibroblasts. Injection of GTP gamma S inhibited ruffling and increased spreading, suggesting an increase in adhesion. To further investigate this, we made use of GRGDSP, a peptide which inhibits binding of integrins to vitronectin and fibronectin. XTC fibroblasts injected with non-hydrolyzable analogs of GTP took much more time to round up than mock-injected cells in response to treatment with GRGDSP, while GDP beta S-injected cells rounded up in less time than controls. Injection with GTP gamma S did not inhibit cell rounding induced by trypsin however, showing that cell contractility is not significantly affected by the activation of GTPases. These data provide evidence for the existence of a GTPase which can control cell-substrate adhesion from the cytoplasm. Treatment of XTC fibroblasts with the phorbol ester 12-o-tetradecanoylphorbol-13-acetate reduced cell spreading and accelerated cell rounding in response to GRGDSP, which is essentially opposite to the effect exerted by non-hydrolyzable GTP analogs. These results suggest the existence of at least two distinct pathways controlling cell-substrate adhesion in XTC fibroblasts, one depending on a GTPase and another one involving protein kinase C.

Neuromodulation

The ability of the nervous system to respond to the environment and to learn depends upon the tuning of neuronal electrical activity, loosely called neuromodulation. The substrates for electrical activity and, therefore, neuromodulation are ion channels which may be either synaptic or extrasynaptic. Neuromodulation is dynamic and most frequently involves neurotransmitters and hormones acting via G-protein-coupled pathways.

Nucleoside diphosphate kinase/NM23 expression in breast cancer: lack of correlation with lymph-node metastasis

The product of the nm23-H1 gene, reported to be a metastatic suppressor gene, was recently identified as the nucleoside diphosphate (NDP) kinase A, and was found to be overexpressed in several types of malignant tumors as compared with normal tissues. In order to determine whether NDP-kinase expression serves as a marker for metastatic potential and whether hyperproliferation of neoplastic cells would correlate with expression, we analyzed NDP-kinase levels and activity by immunohistochemical staining and by an enzymatic assay in 13 benign and 98 malignant breast-tissue specimens. Our results confirm that NDP-kinase expression increases in malignant cells of breast carcinomas, but cannot be considered as a biological marker of metastatic dissemination. No correlation was found between NDP-kinase activity and S phase, taken as an index of cell proliferation. Moreover, no correlation was observed between NDP-kinase activity and tumor size, histoprognostic index, estrogen receptors or progesterone receptors. The mechanism of over-expression of NDP in malignant cells and its role in tumor progression remain to be determined.

Dynamin is a GTPase stimulated to high levels of activity by microtubules

Dynamin was initially identified in calf brain tissue as a protein of relative molecular mass 100,000 which induced nucleotide-sensitive bundling of microtubules. Purified dynamin showed only trace ATPase activity. But in combination with an activating factor removed during the purification, it exhibited microtubule-activated ATPase activity and dynamin-induced bundles showed evidence of ATP-dependent force production. Dynamin is the product of the Drosophila gene shibire, which has been implicated in synaptic vesicle recycling and, more generally, in the budding of endocytic vesicles from the plasma membrane. Dynamin also shows extensive homology with proteins that participate in vacuolar protein sorting and spindle pole-body separation in yeast, and in interferon-induced viral resistance in mammals. All members of this family contain consensus sequence elements consistent with GTP binding near their amino termini, although none has been shown to have GTPase activity. We report here that dynamin is a specific GTPase which can be stimulated to very high levels of activity by microtubules.

Overexpression of nucleoside diphosphate kinase (Nm23) in solid tumours

The product of the nm23-H1 gene, reported to be related to the metastatic potential of tumour cells, was recently identified as the nucleoside diphosphate (NDP) kinase A (Gilles et al., 1991, J Biol Chem, 266, 8784-8789). An analysis of the enzyme by activity measurement and immunological techniques using polyclonal antibodies raised against the NDP kinase A purified from human erythrocytes, was performed on 39 human tissue specimens. Markedly increased activity and higher level of the protein were observed in extracts of solid tumours as compared to the corresponding normal tissues (P less than 0.01). An intense immunolabelling of tumoral cells was observed in sections of the malignant tumours and of some but not all benign neoplasia. The staining is observed in noninvasive and invasive ductal breast carcinomas with or without lymph node involvement as well as in colon and cervix carcinomas and in a case of metastatic melanoma. Therefore, NDP kinase A level is increased in neoplastic tissues but no correlation with metastatic potential could be demonstrated.