Mapping of the catalytic and epitopic sites of human CD38/NAD+ glycohydrolase to a functional domain in the carboxyl terminus

We reported that 1) ecto-NAD+ glycohydrolase (NADase) activity induced upon differentiation of HL-60 cells is localized on the extracellular carboxyl-terminal side of CD38 and that 2) CD38 ligation by specific mAbs is followed by protein tyrosine phosphorylation in the cells. The strategy selected for identifying the relevant catalytic domains of the molecule relies upon the production in COS-7 cells of carboxyl-terminal deletion mutants of CD38. The mutants with fewer than 15 amino acids deleted at the carboxyl terminus of the 300-amino acid wild-type molecule maintained NADase activity, whereas those with more than 27 amino acids deleted did not. The general inference is that the carboxyl-terminal 273-285 sequence bears the site of enzyme activity. Introduction of site-directed mutation of a conserved cysteine residue (Cys275), located in the 273-285 sequence, completely abolished NADase activity. The second issue resolved in this work is the definition of an epitope of the agonistic anti-CD38 mAbs. To this aim, a panel of selected anti-CD38 mAbs was tested using these mutants and various CD38 fragments as the target in immunoblot analyses. All of the epitopes recognized by mAbs inducing protein tyrosine phosphorylation were mapped on an identical site containing the carboxyl-terminal sequence of 273-285. The conclusion is that the discrete carboxyl-terminal sequence identified in the present study not only plays a key role in its ecto-NADase activity, but actually constitutes the epitopes exploited by the agonistic anti-CD38 mAbs for transmembrane signaling.

Molecular characterization of rat T lymphocyte alloantigen RT6.1 as an ADP-ribosyltransferase

A family of glycosylphosphatidylinositol-linked ADP-ribosyltransferases, of which cDNAs were cloned from various mammalian cells, possess a common Glu-rich motif (EEEVLIP) near their carboxyl termini. Although the first Glu in the common motif is replaced by Gln (Q207EEVLIP) in rat T lymphocyte alloantigens RT6.1 and RT6.2, the two RT6s appear to have ADP-ribosyltransferase activity. To investigate the significance of the Glu-rich motif in the enzyme activity, we produced a mutant RT6.1, in which Gln207 was replaced by Glu (Q207E), together with wild-type RT6s, in Escherichia coli. The recombinant RT6.1 and RT6.2 displayed extremely low auto-ADP-ribosylation, though the latter modification was somewhat higher than the former one. In contrast, much higher the auto-modification was observed for Q207E mutant. Moreover, the mutant could effectively ADP-ribosylate agmatine as a substrate. Thus, the single amino acid mutation of RT6.1 caused remarkable increase in its ADP-ribosyltransferase activity, indicating that the Glu-rich motif near the carboxy terminus plays an important role in the enzyme activity.

Signal transduction via the CD38/NAD+ glycohydrolase

The human cell surface CD38 molecule is a 46-kDa type-II transmembrane glycoprotein with a short N-terminal cytoplasmic domain and a long Cys-rich C-terminal extracellular one. We previously demonstrated that an ecto-form NAD+ glycohydrolase (NADase) activity induced by all-trans retinoic acid in HL-60 cells is due to the extracellular domain of CD38. In the present study, we investigated a possible signal transduction mediated through CD38 in the retinoic acid-differentiated HL-60 cells with anti-CD38 monoclonal antibodies (mAbs). The addition of selected anti-CD38 mAbs to the cells induced rapid tyrosine phosphorylation of the cellular proteins with the molecular weights of 120,000, 87,000 and 77,000; the phosphorylated 120-kDa protein was identified as the c-cbl proto-oncogene product, p120c-cbl. Furthermore, the phosphorylated p 120c-cbl associated with the 85-kDa subunit of phosphatidylinositol 3-kinase. To determine the relationship between the amino acid sequence responsible for the NADase activity and epitopes recognized by the stimulatory mAbs, we produced its carboxy-terminal deletion mutants in COS-7 cells. The mutants with less than 15 amino acids deleted from the carboxyl terminus of the 300-amino acid wild-type molecule still maintained NADase activity, but those with more than 27 amino acids deleted did not. Introduction of site-directed mutation of a cysteine residue (Cys275), located in the 273-285 sequence, completely abolished the NADase activity. These CD38 mutants were also used for an epitope mapping of anti-CD38 mAbs. All the epitopes recognized by the mAbs inducing the tyrosine phosphorylation were mapped on the same Cys275-containing sequence of 273-285. Thus, the discrete carboxy-terminal sequence not only plays a key role in its ecto-NADase activity, but also contains the epitopes of the agonistic anti-CD38 mAbs for the transmembrane signaling. We also found that the agonistic mAbs markedly potentiate superoxide generation induced by the stimulation of G protein-coupled chemotactic receptors. Our results suggested that the stimulation of CD38 might generate an accessory signal(s) to enhance the G protein-mediated signaling, probably though the protein-tyrosine phosphorylation.

Association of phosphatidylinositol 3-kinase with the proto-oncogene product Cbl upon CD38 ligation by a specific monoclonal antibody in THP-1 cells

We reported that ecto-NAD+ glycohydrolase activity induced upon differentiation of HL-60 cells with retinoic acid is localized on the extracellular domain of CD38 and that CD38 ligation by a specific monoclonal antibody, HB-7, is followed by rapid tyrosine phosphorylation of cellular proteins including a proto-oncogene product, Cbl. In the present study, we investigated intracellular signaling linked to the HB-7-induced Cbl phosphorylation in dibutyryl cAMP-treated THP-1 cells. The 85-kDa regulatory subunit (p85) of phosphatidylinositol (PI) 3-kinase was immunoprecipitated with anti-Cbl antibody in a manner dependent on the tyrosine phosphorylation of Cbl. PI 3-kinase activity was also observed in the immunoprecipitated fractions containing tyrosine-phosphorylated Cbl. The phosphorylated form of Cbl, which had been separated from the CD38-stimulated cells, was capable of directly binding to a recombinant p85 fused to glutathione S-transferase. Thus, the direct association of tyrosine-phosphorylated Cbl with PI 3-kinase, possibly leading to the kinase activation, appeared to be involved in intracellular signaling caused by the CD38 ligation.

Involvement of the beta gamma subunits of inhibitory GTP-binding protein in chemoattractant receptor-mediated potentiation of cyclic AMP formation in guinea pig neutrophils

Cellular cyclic AMP formation in response to prostaglandin (PG) E1 was markedly potentiated by the chemoattractant formyl-Met-Leu-Phe (fMLP) in guinea pig neutrophils. This potentiation by fMLP was abolished by prior treatment of the cells with pertussis toxin, but not by the prevention of an fMLP-induced intracellular Ca2+ increase in the cells, indicating the direct involvement of the inhibitory GTP-binding protein (Gj), but not Ca2+, in the fMLP-induced potentiation of cyclic AMP formation. Cyclic AMP formation in the neutrophils was also unique in response to forskolin; the diterpene inhibited cyclic AMP formation stimulated by PGE1 plus fMLP at low concentrations, but it slightly stimulated the basal and fMLP-induced cyclic AMP formation at high concentrations. Such a forskolin-induced inhibition was also observed in the adenylyl cyclase of the cell membranes and detergent extract therefrom only when the cyclase was activated by GTP or its nonhydrolyzable analogue (GTP gamma S). The forskolin-inhibitable activity could be affinity-purified from the GTP gamma S-treated cell membranes with a forskolin-agarose column. The cyclase appeared to be purified as a complex with the GTP gamma S-bound alpha subunit of the stimulatory GTP-binding protein (Gs alpha), but not with the beta gamma subunits, as judged from immunoblot analysis with specific antisera. The GTP gamma S-bound Gs alpha-stimulated cyclase activity was further enhanced by beta gamma, and this enhancement was again inhibited by forskolin. These results suggest that the GTP-bound Gs alpha produced by PGE1 receptor stimulation and the beta gamma subunits released from Gj by fMLP receptor stimulation were acting synergistically in the cyclic AMP formation of intact neutrophils.

Intrinsic signaling function of APP as a novel target of three V642 mutations linked to familial Alzheimer's disease

APP695 is a transmembrane precursor of Abeta amyloid. In familial Alzheimer's disease (FAD), three mutations V642I/F/G were discovered in APP695, which has been suggested by multiple studies to be a cell surface signaling receptor. We previously reported that normal APP695 encodes a potential GO-linked receptor with ligand-regulated function and that expression of the three FAD mutants (FAD-APPs), not normal APP, induces cellular outputs by GO-dependent mechanisms. This suggests that FAD-APPs are constitutively active GO-linked receptors. Here, we provide direct evidence for this notion. Reconstitution of either recombinant FAD-APP with GO vesicles induced activation of GO, which was inhibitable by pertussis toxin, sensitive to Mg2+ and proportional in quantity to the reconstituted amounts of FAD-APP. Consistent with the dominant inheritance of this type of FAD, this function was dominant over normal APP, because little activation was observed in APP695-GO vesicles. Experiments with antibody competition and sequence deletion indicated that His657-Lys676 of FAD-APP, which has been specified as the ligand-dependent GO-coupling domain of normal APP, was responsible for this constitutive activation, confirming that the three FAD-APPs are mutationally activated APP695. This study identifies the intrinsic signaling function of APP to be a novel target of hereditary Alzheimer's disease mutations, providing an in vitro system for the screening of potential FAD inhibitors.

Synergistic activation of PtdIns 3-kinase by tyrosine-phosphorylated peptide and beta gamma-subunits of GTP-binding proteins

Stimulation of differentiated THP-1 cells by insulin led to rapid accumulation of PtdIns(3,4,5)P3, a product of PtdIns 3-kinase. Stimulation of the GTP-binding-protein-linked receptor by N-formylmethionyl-leucyl-phenylalanine (fMLP) also induced the accumulation of PtdIns(3,4,5)P3 in the cells. The effect of insulin was, while that of fMLP was not, accompanied by increased PtdIns 3-kinase activity in the anti-phosphotyrosine immuno-precipitate. The combination of insulin and fMLP induced more PtdIns(3,4,5)P3 production than the sum of the individual effects. The insulin-induced recruitment of PtdIns 3-kinase activity in the anti-phosphotyrosine immunoprecipitate was unaffected by the combined treatment with fMLP. To investigate the mechanism underlying the synergistic accumulation of PtdIns(3,4,5)P3, we separated the cytosolic proteins of THP-1 cells on a Mono Q column. PtdIns 3-kinase activities were eluted in two peaks, and one of the peaks markedly increased on the addition of beta gamma-subunits of GTP-binding proteins (G beta gamma). The other peak was affected only slightly by G beta gamma, but was synergistically increased by G beta gamma and a tyrosine-phosphorylated peptide which was synthesized accordingly to the amino acid sequence of insulin receptor substrate-1. The activity in the latter fraction was completely immunoprecipitated by an antibody against the regulatory subunit of PtdIns 3-kinase (p85). These results suggest that the conventional PtdIns 3-kinase (p85/p110), which has been implicated in insulin-induced cellular events, or a closely related isoenzyme is controlled by a combination of a tyrosine-phosphorylated protein and a GTP-binding protein in intact cells.

Stimulation of ADP-ribosyl cyclase activity of the cell surface antigen CD38 by zinc ions resulting from inhibition of its NAD+ glycohydrolase activity

The lymphocyte cell surface antigen, CD38, which has an amino acid sequence similar to Aplysia ADP-ribosyl cyclase, catalyzes not only the hydrolysis of NAD+ and 1-(5-phospho-beta-D-ribosyl) adenosine 5'-phosphate cyclic anhydride (cyclic ADP-ribose) but also the formation of cyclic ADP-ribose from NAD+. To characterize the bifunctional enzyme properties, we produced the recombinant CD38 fused with a maltose-binding protein (MBP-CD38). Zinc ions stimulated the ADP-ribosyl cyclase activity of MBP-CD38, but inversely inhibited its NAD+ glycohydrolase activity which was approximately 100-fold dominant to the cyclase activity in the absence of Zn2+. Such dual effects of Zn2+ were also observed in the native membrane-bound CD38 of HL-60 cells which had been caused to differentiate by retinoic acid. Zinc ions inhibited the NAD+ glycohydrolase reaction catalyzed by MBP-CD38 in an uncompetitive manner, whereas they enhanced the ADP-ribosyl cyclase reaction without affecting the Km value for NAD+. There was an increase in the fluorescence intensity of a hydrophobic fluorescent probe, 8-anilino-1-naphthalenesulfonate, in the presence of MBP-CD38. The fluorescence increase was further enhanced by the addition of Zn2+ with a shift in the maximum emission wavelength from 484 nm to 470 nm, suggesting that Zn2+ caused conformational changes of MBP-CD38. These results indicate that Zn2+ directly interacts with CD38 to stimulate its ADP-ribosyl cyclase with inhibition of its NAD+ glycohydrolase, probably due to prevention of the access of water molecule to an intermediate of the enzymesubstrate complex.

GTP-binding protein activation underlies LTP induction by mast cell degranulating peptide

Mast cell degranulating peptide (MCD) induces long-term potentiation (LTP) in the CA1 region of the hippocampus. MCD has been shown to bind to a voltage-dependent A-type potassium channel with high-affinity (less than 1 nM). However, the concentration necessary to induce LTP is more than 500 nM, suggesting that some other functions of MCD are also fundamental to LTP induction. The concentration of MCD required for LTP induction was greatly reduced by preactivating G proteins. This fact suggests that G protein activation by MCD also plays an important role in LTP induction. MCD-binding proteins were purified from rat brain. G proteins were found to exist in a non-denatured state in this affinity-purified fraction. When reconstituted into a planar lipid bilayer membrane, a potassium-selective and voltage-dependent current could be observed. This channel was blocked by MCD at a high concentration equal to the effective concentration for G protein activation. Addition of GTP-gamma-S significantly blocked the reconstituted current. Thus, we identified a pathway for LTP induction by MCD in which high concentrations of MCD activate G protein which in turns leads to blocking of a potassium channel.

[The evaluation of three dimensional image distortion of helical scanning CT]

We developed a computer algorithm to simulate distortion in the three-dimensional (3D) displays obtained by helical scanning. The algorithm constitutes calculation of the image profile in the longitudinal direction, which is assumed to be a convolution of the object function with the slice sensitivity profile (SSP) of helical scanning. Experiments were performed to examine the algorithm for its validity with the use of a K2HPO4 phantom. Simulated results and measurements was in a good agreement. The distortion was investigated by the computer simulation. The model simulated was a high density object (Ol) surrounded by low density tissue (Os). The helical interpolation used was 360-degree linear interpolation. Two parameters were defined: delta Lz, which is the difference in length between the 3D image and the actual object Ol in the longitudinal direction, and the cut level index (CLI), which is defined as CLI = (Cut Level CT Number-CT Number of Os)/(CT Number of OI-CT Number of Os). We found that magnitude of delta Lz increased depending on the table feed distance per 360-degree scan (Dt). When Ol was twice as long as Dt, delta Lz directly depended on CLI, but was independent of Ol length. When Ol length was longer than Dt, delta Lz was shown to be 0 at CLI not equal to 0.5 at every Dt. When Ol was shorter than Dt, delta Lz decreased remarkably depending on the length of Ol in higher CLI. The simulations, with the use of a newly developed algorithm, were demonstrated to be useful for evaluating the amount of distortion and for better understanding the characteristics of 3D displays of helical scanning.

Increase in ADP-ribosyltransferase activity of rat T lymphocyte alloantigen RT6.1 by a single amino acid mutation

A family of glycosylphosphatidylinositol-linked ADP-ribosyltransferases, of which cDNAs were cloned from various mammalian cells, possess a common Glu-rich motif (EEEVLIP) near their carboxyl termini. Although the first Glu in the common motif is replaced by Gln (Q207EEVLIP) in rat T lymphocyte alloantigens RT6.1 and RT6.2, the two RT6s appear to have both activities of NAD+ glycohydrolase and ADP-ribosyltransferase to a lesser extent. To investigate the significance of the Glu-rich motif in the two enzyme activities, we produced a mutant RT6.1 (Q207E), in which Gln207 was replaced by Glu, together with wild-type RT6s, in Escherichia coli. Kinetic analysis revealed that there were no marked differences in the Vmax and Km values of NAD+ glycohydrolases among the three recombinant proteins. The recombinant RT6.1 and RT6.2 displayed extremely low auto-ADP-ribosylation, although the latter modification was somewhat higher than the former. In contrast, much greater auto-modification was observed for the Q207E mutant. Moreover, the mutant could effectively ADP-ribosylate agmatine as a substrate. Thus, the single amino acid mutation of RT6.1 caused a marked increase in its ADP-ribosyltransferase activity, indicating that the Glu-rich motif near the carboxy terminus plays an important role in the enzyme activity.

Inhibition of NAD+ glycohydrolase and ADP-ribosyl cyclase activities of leukocyte cell surface antigen CD38 by gangliosides

We have recently reported that gangliosides act as inhibitors of ADP-ribosyltransferases and NAD+ glycohydrolases (NADase) of pertussis toxin and the C3 exoenzyme from Clostridium botulinum (Hara-Yokoyama, M., Hirabayashi, Y., Irie, F., Syuto, B., Moriishi, K., Sugiya, H., and Furuyama, S. (1995) J. Biol. Chem. 270, 8115-8121). Here, we investigated the effect of gangliosides on the enzymatic activity of leukocyte cell surface antigen CD38, which is identified as an ecto-NADase (Kontani, K., Nishina, H., Ohoka, Y., Takahashi, K., and Katada, T. (1993) J. Biol. Chem. 268, 16895-16898). Gangliosides GM1a and GQ1balpha inhibited the NADase activity in the immunoprecipitate of anti-CD38 antibody from the membrane extract of retinoic acid-treated human leukemic HL-60 cells. Gangliosides also inhibited the NADase activity of the extracellular domain of CD38 antigen that was deprived of the transmembrane domain and was expressed in Escherichia coli as a fusion protein with maltose-binding protein (MBP-CD38). The order of the inhibitory effect of purified ganglioside species on the NADase activity on MBP-CD38 was as follows: GQ1balpha > GT1b, GQ1b > GD1a, GD1b, GM1a, GM1b, GD3, GM3. GQ1balpha inhibited the NADase of MBP-CD38 in a noncompetitive manner versus NAD+ with a Ki value of about 0.3 microM. Neither ceramide nor the oligosaccharide moiety of GQ1balpha had an effect on the NADase activity. GQ1balpha, GT1b, and GQ1b also efficiently inhibited the ADP-ribosyl cyclase activity of MBP-CD38. At present, gangliosides are the only endogenous species that can block the enzymatic activity of CD38 antigen. The present results suggest a potential role of gangliosides as inhibitors of the ecto-NADases.

Induction of CD38/NADase and its monoclonal antibody-induced tyrosine phosphorylation in human leukemia cell lines

We previously reported that CD38 characterized as an ecto-enzyme of NAD+ glycohydrolase (NADase) was specifically induced by retinoic acid (RA) in human promyelocytic leukemia HL-60 cells and that anti-CD38 monoclonal antibody (mAb) induced tyrosine phosphorylation of cellular proteins in the RA-differentiated cells. In the present study, we found that CD38/NADase was induced in human monocytic leukemia THP-1 cells not only by RA but also by dibutyryl cAMP, which had no effect on the induction of CD38 in HL-60 cells. Similarly in the RA-differentiated HL-60 cells, tyrosine phosphorylation by anti-CD38 mAb was also observed in the CD38-expressing THP-1 cells, regardless of whether CD38 was induced by RA or dibutyryl cAMP. Such tyrosine phosphorylation was, however, not observed in human lymphoblastic leukemia Jurkat cells which had been treated by RA to produce CD38. Thus, the induction of CD38/NADase appeared to be not limited for RA-dependent differentiation and not always linked to protein-tyrosine phosphorylation in human leukemic cell lines.

Inositol 1,4,5-trisphosphate-gated calcium transport through plasma membranes in nerve terminals

We developed new biochemical approaches to demonstrate the presence of inositol 1,4,5-triphosphate (InsP3)-gated calcium channels in presynaptic plasma membranes (SPM) and their involvement in the presynaptic receptor-mediated Ca2+ influx into nerve terminals. In perfusion experiments using SPM vesicles preloaded with 45Ca2+, InsP3 elicited the release of 45CA2+ into perfusates in a saturable manner. The InsP3- evoked 45Ca2+ release from resealed SPM vesicles was more potent than that from resealed vesicles using any other subcellular fractions. Here we also report the involvement of InsP3-gated mechanisms in the presynaptic receptor-mediated Ca2+ influx into synaptosomes (nerve terminals) by use of such resealed vesicles reconstituted with purified Gi1.

Focal adhesion kinase (p125FAK) and paxillin are substrates for sphingomyelinase-induced tyrosine phosphorylation in Swiss 3T3 fibroblasts

We examined the effect of sphingomyelinase on tyrosine phosphorylation of intracellular proteins in mouse Swiss 3T3 fibroblasts. Incubation of the cells with bacterial sphingomyelinase resulted in the elevation of tyrosine phosphorylation of multiple cellular proteins of 190, 130, 120, 97 and 70 kDa within minutes. The 120 and 70 kDa tyrosine-phosphorylated peptides were identified as p125 focal adhesion kinase (p125FAK) and paxillin respectively by the use of specific antibodies against the proteins. Tyrosine kinase activity associated with anti-p125FAK immunoprecipitate was stimulated by incubation of cells with sphingomyelinase. Cytochalasin D, which selectively disrupts the network of actin filaments, inhibited sphingomyelinase-induced tyrosine phosphorylation of p125FAK and elevation of tyrosine kinase activity in the anti-p125FAK immunoprecipitates. Sphingomyelinase-induced phosphorylation of p125FAK was not inhibited by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. This was in sharp contrast with a wortmannin-sensitive phosphorylation of p125FAK observed in platelet-derived growth factor (PGDF)-stimulated cells. Thus hydrolysis of sphingomyelin is considered to regulate the tyrosine kinase cascade including p125FAK and paxillin by a mechanism distinct from PDGF.

Activation of phosphatidylinositol 3-kinase by concanavalin A through dual signaling pathways, G-protein-coupled and phosphotyrosine-related, and an essential role of the G-protein-coupled signals for the lectin-induced respiratory burst in human monocytic THP-1 cells

Stimulation of monocytic THP-1 cells by a lectin, concanavalin A (Con A), resulted in protein-tyrosine phosphorylation and association of some of the thus phosphorylated proteins with the 85 kDa regulatory subunit of PtdIns 3-kinase. Both actions of Con A were not inhibited by wortmannin, a PtdIns 3-kinase inhibitor, or by prior exposure of cells to pertussis toxin which uncouples certain G-proteins from receptors. The binding of PtdIns 3-kinase to the tyrosine-phosphorylated proteins increased upon Con A stimulation; there was a marked increase in the enzymic activity in the anti-phosphotyrosine immuno-precipitates from Con A-treated cells. The increase was abolished by wortmannin but not affected by pertussis toxin. The incorporation of 32P into PtdInsP3 also increased during incubation of [32P]P(i)-prelabelled cells with Con A, reflecting activation of whole-cell PtdIns 3-kinase which could not be accounted for solely by the increase in the phosphotyrosine-bound enzyme activity from the following aspects: (1) different concentration dependencies for Con A; and (2) almost total susceptibility of the incorporation to pertussis toxin. This notion appears to be supported by different time courses between increases in PtdInsP3 production and the phosphotyrosine-bound activity. The susceptibility to the toxin may reflect involvement of the toxin-sensitive G-proteins. In contrast, insulin-induced increases in PtdInsP3 production, as well as increases in phosphotyrosine-bound PtdIns 3-kinase activity, were blocked by wortmannin, but never affected by prior exposure of cells to pertussis toxin, excluding a possible involvement of G-proteins in the insulin-induced activation. Con-A-induced O2- production was almost inhibited by either pertussis toxin or wortmannin. These results suggest that oligomerization of cell-surface glycoproteins with Con A gives rise to activation of G-protein(s) and certain tyrosine kinase(s), both of which were responsible for PtdIns 3-kinase activation; the G-protein-mediated activation led to the respiratory burst.

Specific association of phosphatidylinositol 3-kinase with the protooncogene product Cbl in Fc gamma receptor signaling

A tyrosine-phosphorylated protein with a molecular mass of 115 kDa was reported to be tightly associated with the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase, when the enzyme was essentially activated upon ligand engagement of Fc gamma receptors (Fc gamma R) leading to engulfment of IgG-coated erythrocytes by phagocytes [Ninomiya et al. (1994) J. Biol. Chem. 269, 22732-22737]. Here, the 115-kDa protein is identified as the product of human c-cbl protooncogene. Cross-linking of Fc gamma RII on the surface of THP-1 cells triggered (a) prominent tyrosine phosphorylation of Cbl, (b) activation of PI 3-kinase that was immunoprecipitated with the anti-Cbl or the anti-phosphotyrosine antibody, and (c) specific association of Cbl with p85. Thus, Cbl functions in phagocytes as a result of its association with PI 3-kinase in response to Fc gamma R ligation.

Partially purified RhoA-stimulated phospholipase D activity specifically binds to phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PIP2) is absolutely required for the ADP-ribosylation factor-stimulated phospholipase D (PLD) activity. In the present study, partially purified rat brain PLD was found to be activated by another PLD activator, RhoA, when PIP2, but not other acidic phospholipids, was included in vesicles comprising phosphatidylethanolamine (PE) and the PLD substrate phosphatidyicholine (PC) (PE/PC vesicles), demonstrating the absolute requirement of PIP2 for the RhoA-stimulated PLD activation, too. It is interesting that the RhoA-dependent PLD activity in the partially purified preparation was drastically decreased after the preparation was incubated with and separated from PE/PC vesicles containing PIP2. The PLD activity was extracted by higher concentrations of NaCl from the vesicles containing PIP2 that were incubated with and then separated from the partially purified PLD preparation. These results demonstrate that RhoA-dependent PLD binds to PE/PC vesicles with PIP2. The degree of binding of the RhoA-dependent PLD activity to the vesicles was totally dependent on the amount of PIP2 in the vesicles and correlated well with the extent of the enzyme activation. Further-more, it was found that a recombinant peptide of the pleckstrin homology domain of beta-adrenergic receptor kinase fused to glutathione S-transferase, which specifically binds to PIP2, inhibited the PIP2-stimulated, RhoA-dependent PLD activity in a concentration-dependent manner. From these results, it is concluded that in vitro rat brain PLD translocates to the vesicles containing PIP2, owing to its specific interaction with PIP2, to access its substrate PC, thereby catalyzing the hydrolysis of PC. PLD appears to localize exclusively on plasma membranes of cells and tissues. An aminoglycoside, neomycin, that has high affinity for PIP2 effectively extracted the RhoA-dependent PLD activity from rat brain membranes. This indicates that PIP2 serves as an anchor to localize PLD on plasma membranes in vivo.

Augmentation by calmodulin of ADP-ribosylation factor-stimulated phospholipase D activity in permeabilized rabbit peritoneal neutrophils

Activation of the membrane-bound phospholipase D (PLD) requires cytosolic factor(s), and ADP-ribosylation factor (ARF) has been identified as a cytosolic PLD activator. In the present study, we demonstrate that calmodulin (CaM) and ARF are both involved in PLD activation in rabbit peritoneal neutrophils. The PLD activity of streptolysin O-permeabilized, cytosol-depleted rabbit neutrophils was significantly enhanced when the permeabilized cells were reconstituted with bovine brain cytosol in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), whereas there was little activation of the enzyme in the absence of cytosol. The GTP gamma S-stimulated PLD activity in the presence of cytosol was augmented on increasing the concentration of free Ca2+. The PLD activity stimulated by GTP gamma S and Ca2+ in this system was inhibited by the calmodulin inhibitor W-7. These findings suggest that CaM plays a role as a cytosolic PLD activator. Moreover, highly purified CaM alone, as well as partially purified ARF alone, promoted a slight stimulation of the PLD activity in permeabilized neutrophils. Interestingly, ARF-stimulated PLD activity was augmented by CaM in the presence of GTP gamma S and Ca2+. This augmentation was again inhibited by W-7, as well as by the structurally unrelated CaM inhibitor trifluoperazine. These data imply that CaM stimulates the PLD activity of rabbit neutrophils in concert with ARF.

A subtype of kappa-opioid receptor mediates inhibition of high-affinity GTPase inherent in Gi1 in guinea pig cerebellar membranes

The kappa-opioid receptor agonists including U-50,488H and dynorphin A (1-17) in ranges of 0.1-100 nM inhibited the hydrolysis of GTP to GDP (P(i) release) inherent in GTP-binding proteins (G proteins) in guinea pig cerebellar membranes. U-50,488H inhibited only high-affinity GTPase activity, not low-affinity activity. The action of this agonist was found to be biphasic, and there was no inhibition at concentrations > 1 microM. The inhibition was abolished by pretreatment with preactivated pertussis toxin (PTX) at concentrations > 1 micrograms/ml but not with preactivated cholera toxin (30 micrograms/ml). Similar blockade of kappa-receptor-mediated inhibition was also observed when membranes were pretreated with a low concentration (8 microM) of N-ethylmaleimide (NEM) at low temperature (4 degrees C), which alkylates the cysteine residue to be ADP-ribosylated by PTX; but this treatment caused no significant change in kappa-agonist binding. When purified Gi1, but not G(o), was reconstituted into membranes pretreated with NEM, the kappa-receptor-mediated inhibition was recovered. These findings suggest that a subtype of kappa-opioid receptor is coupled to inhibition of intrinsic activity of Gi1.

Augmentation by calmodulin of ADP-ribosylation factor-stimulated phospholipase D activity in permeabilized rabbit peritoneal neutrophils

Activation of the membrane-bound phospholipase D (PLD) requires cytosolic factor(s), and ADP-ribosylation factor (ARF) has been identified as a cytosolic PLD activator. In the present study, we demonstrate that calmodulin (CaM) and ARF are both involved in PLD activation in rabbit peritoneal neutrophils. The PLD activity of streptolysin O-permeabilized, cytosol-depleted rabbit neutrophils was significantly enhanced when the permeabilized cells were reconstituted with bovine brain cytosol in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), whereas there was little activation of the enzyme in the absence of cytosol. The GTP gamma S-stimulated PLD activity in the presence of cytosol was augmented on increasing the concentration of free Ca2+. The PLD activity stimulated by GTP gamma S and Ca2+ in this system was inhibited by the calmodulin inhibitor W-7. These findings suggest that CaM plays a role as a cytosolic PLD activator. Moreover, highly purified CaM alone, as well as partially purified ARF alone, promoted a slight stimulation of the PLD activity in permeabilized neutrophils. Interestingly, ARF-stimulated PLD activity was augmented by CaM in the presence of GTP gamma S and Ca2+. This augmentation was again inhibited by W-7, as well as by the structurally unrelated CaM inhibitor trifluoperazine. These data imply that CaM stimulates the PLD activity of rabbit neutrophils in concert with ARF.

Tyrosine phosphorylation of the c-cbl proto-oncogene product mediated by cell surface antigen CD38 in HL-60 cells

The human cell surface antigen CD38 is a 46-kDa type II transmembrane glycoprotein with a short N-terminal cytoplasmic domain and a long Cys-rich C-terminal extracellular one. We demonstrated previously that the extracellular domain of CD38 has NAD+ glycohydrolase (NADase) activity and that the ecto-form NADase activity induced in HL-60 cells during cell differentiation by retinoic acid is due to CD38. In the present study, we investigated the intracellular signaling mediated by CD38 in retinoic acid-differentiated HL-60 cells with an anti-CD38 monoclonal antibody. The addition of anti-CD38 monoclonal antibody to the cells induced rapid tyrosine phosphorylation of the cellular proteins with molecular weights of 120,000, 87,000, and 77,000. An increase in tyrosine kinase activity in the anti-phosphotyrosine immunoprecipitates of the cells was also observed after the addition of anti-CD38 monoclonal antibody. Moreover, one of the prominent tyrosine-phosphorylated proteins stimulated by the anti-CD38 monoclonal antibody was identified as the c-cbl proto-oncogene product, p120c-cbl. These results indicated that tyrosine phosphorylation of cellular proteins, including p120c-cbl, is possibly involved in transmembrane signaling mediated by CD38.

Radiolabeling of catalytic subunits of PI 3-kinases with 17 beta-hydroxy-16 alpha(-)[125I]iodowortmannin: identification of the G beta gamma-sensitive isoform as a complex composed of 46-kDa and 100-kDa subunits

A fungal metabolite, wortmannin, is a potent inhibitor of phosphatidylinositol (PI) 3-kinases. In the present study, we prepared a radiolabeled derivative of wortmannin, 17 beta-hydroxy-16 alpha(-)[125I]iodowortmannin. The compound bound tightly to a 110-kDa subunit in the previously identified isoform of PI 3-kinase (p85/p110), and also to a 100-kDa peptide in a partially purified preparation of another isoform of PI 3-kinase whose activity was markedly stimulated by the beta gamma subunits of GTP-binding proteins (G beta gamma). The binding to both peptides was inhibited by non-radiolabeled wortmannin and also by LY294002, another inhibitor of PI 3-kinases. An antibody against p85 recognized a 46-kDa peptide in the G beta gamma-sensitive isozyme and precipitated the 100-kDa peptide specifically labeled with 17 beta-hydroxy-16 alpha(-)[125I]iodowortmannin. These results suggested that the newly found isozyme was a complex composed of 46-kDa and 100-kDa peptides.

Permissive effect of ceramide on growth factor-induced cell proliferation

Addition of bacterial sphingomyelinase to quiescent Swiss 3T3 cells effectively potentiated the platelet-derived growth factor (PDGF)-stimulated cell proliferation, though the enzyme by itself had little effect on the cell proliferation. Such potentiation of the cell growth could also be observed by the addition of ceramide, a product of the sphingomyelinase-catalysed reaction. In contrast, phosphocholine, another product of the enzyme reaction, had no synergistic effect on the action of PDGF. Treatment of the cells with sphingomyelinase or ceramide increased the cellular activity of mitogen-activated protein kinases (MAP kinases), which have been implicated in the regulation of cell proliferation. However, the synergistic effect of sphingomyelinase on the PDGF-induced cell growth could still be observed even when the cellular MAP kinase activity was fully activated by the growth factor alone. These results indicate that a ceramide-mediated cellular event(s) other than the MAP kinase activation is potentially involved in the regulation of cell growth.

Significance of Thr182 in the nucleotide-exchange and GTP-hydrolysis reactions of the alpha subunit of GTP-binding protein Gi2

The crystal structures of the GTP- and GDP-bound alpha subunits of heterotrimeric GTP-binding proteins were recently determined, and a conserved Thr residue in the G2 (linker 2) region of the alpha subunits, which corresponds to Thr182 in Gi2 alpha, was deduced to interact with the gamma-phosphate of GTP and Mg2+. To investigate biochemically the significance of the Thr residue, we produced a mutant Gi2 alpha, in which Thr182 was substituted for Ala (T182A), in Escherichia coli. The rate of guanosine 5'-(gamma-thio)tri-phosphate (GTP gamma S) binding to T182A was higher than that to the wild-type Gi2 alpha, especially with a high concentration (10 mM) of Mg2+. The rate of dissociation of bound GDP from T182A was also much faster than that from the wild-type with the high Mg2+ concentration. Moreover, T182A had much lower GTPase activity than the wild-type, like the gip mutant (R179C) of Gi2 alpha found in human endocrine tumors. The ability of T182A to interact with beta gamma subunits and membrane-bound receptors was the same as that of the wild-type alpha subunit. T182A could take on a GTP-bound active conformation, as judged from its sensitivity to tryptic digestion. These results indicated that Thr182 plays an important role not only in the Mg(2+)-sensitive GDP-GTP exchange reaction but also in the GTPase activity of Gi2 alpha. The T182A mutant of Gi2 alpha, characterized by the faster GDP release and the slower GTP hydrolysis, would be a novel mutant that retains the ability to interact with receptors and beta gamma subunits.

Neuronal localization of CD38 antigen in the human brain

CD38 is a lymphocyte differentiation antigen which is involved in the cyclic ADP-ribose-mediated second messenger system. We provide immunochemical and immunohistochemical evidence for the expression of CD38 in the adult human brain. We used six polyclonal antibodies against synthetic CD38 polypeptides, in addition to four monoclonal antibodies already available. Brain CD38 was detectable by Western blotting after immunoaffinity purification of the brain extracts. Immunoperoxidase staining localized CD38 immunoreactivity to the perikarya and dendrites of many neurons, such as the cerebellar Purkinje cells, implying that CD38 is involved in the signal transduction within the central nervous system neurons.

Synergistic activation of rat brain phospholipase D by ADP-ribosylation factor and rhoA p21, and its inhibition by Clostridium botulinum C3 exoenzyme

An activator of rat brain phospholipase D (PLD) that is distinct from the already identified PLD activator, ADP-ribosylation factor (ARF), was partially purified from bovine brain cytosol by a series of chromatographic steps. The partially purified preparation contained a 22-kDa substrate for Clostridium botulinum C3 exoenzyme ADP-ribosyltransferase, which strongly reacted with anti-rhoA p21 antibody, but not with anti-rac1 p21 or anti-cdc42Hs p21 antibody. Treatment of the partially purified PLD-activating factor with both C3 exoenzyme and NAD significantly inhibited the PLD-stimulating activity. These results suggest that rhoA p21 is, at least in part, responsible for the PLD-stimulating activity in the preparation. Recombinant isoprenylated rhoA p21 expressed in and purified from Sf9 cells activated rat brain PLD in a concentration- and GTP gamma S (guanosine 5'-O-(3-thiotriphosphate))-dependent manner. In contrast, recombinant non-isoprenylated rhoA p21 (fused to glutathione S-transferase) expressed in Escherichia coli failed to activate the PLD. This difference cannot be explained by a lower affinity of non-isoprenylated rhoA p21 for GTP gamma S, as the rates of [35S]GTP gamma S binding were very similar for both recombinant preparations and the GTP gamma S-bound form of non-isoprenylated rhoA p21 did not induce PLD activation. Interestingly, recombinant isoprenylated rhoA p21 and ARF synergistically activated rat brain PLD; a similar pattern was seen with the partially purified PLD-activating factor. The synergistic activation was inhibited by C3 exoenzyme-catalyzed ADP-ribosylation of recombinant isoprenylated rhoA p21 in a NAD-dependent manner. Inhibition correlated with the extent of ADP-ribosylation. These findings suggest that rhoA p21 regulates rat brain PLD in concert with ARF, and that isoprenylation of rhoA p21 is essential for PLD regulation in vitro.

Cyclic AMP-increasing agents interfere with chemoattractant-induced respiratory burst in neutrophils as a result of the inhibition of phosphatidylinositol 3-kinase rather than receptor-operated Ca2+ influx

Superoxide anion and arachidonic acid were produced in guinea pig neutrophils in response to a chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Both responses were markedly, but the former response to a phorbol ester was not at all, inhibited when the cellular cAMP level was raised by prostaglandin E1 combined with a cAMP phosphodiesterase inhibitor. Increasing cAMP was also inhibitory to fMLP-induced activation of phosphatidylinositol (PI) 3-kinase and Ca2+ influx without any effect on the cation mobilization from intracellular stores. The fMLP-induced respiratory burst was abolished when PI 3-kinase was inhibited by wortmannin or LY294002, but was not affected when Ca2+ influx was inhibited. On the contrary, fMLP released arachidonic acid from the cells treated with the PI 3-kinase inhibitors as well as from non-treated cells, but it did not so when cellular Ca2+ uptake was prevented. The chemotactic peptide activated PI 3-kinase even in cells in which the receptor-mediated intracellular Ca2+ mobilization and respiratory burst were both abolished by exposure of the cells to a permeable Ca(2+)-chelating agent. Thus, stimulation of fMLP receptors gave rise to dual effects, activation of PI 3-kinase and intracellular Ca2+ mobilization; both effects were necessary for the fMLP-induced respiratory burst. Increasing cellular cAMP inhibited the respiratory burst and arachidonic acid release as a result of the inhibitions of PI 3-kinase and Ca2+ influx, respectively, in fMLP-treated neutrophils.

NAD(+)-dependent ADP-ribosylation of T lymphocyte alloantigen RT6.1 reversibly proceeding in intact rat lymphocytes

Rat T lymphocyte alloantigen 6.1 (RT6.1), which was synthesized as the fusion protein with a maltose-binding protein in Escherichia coli, displayed NAD(+)-dependent auto-ADP-ribosylation in addition to an enzyme activity of NAD+ glycohydrolase. Such ADP-ribosylation of RT6.1 was also observed in lymphocytes isolated from rat tissues as follows. When intact rat lymphocytes expressing RT6.1 mRNA were incubated with [alpha-32P]NAD+, its radioactivity was incorporated into a cell surface protein with the M(r) of 31,000. The radiolabeled 31-kDa protein was released from the cell surface by treatment of the cells with phosphatidylinositol-specific phospholipase C and immunoprecipitated with anti-RT6.1 antiserum. The radioactivity incorporated into the 31-kDa protein was recovered as 5'-[32P]AMP upon incubation with snake venom phosphodiesterase and also removed by NH2OH treatment. These results suggested that the NAD(+)-dependent modification of the 31-kDa protein was due to ADP-ribosylation of glycosylphosphatidylinositol-anchored RT6.1 at an arginine residue. When intact lymphocytes, in which RT6.1 had been once modified by [32P]ADP-ribosylation, were further incubated in the absence of NAD+, there was reduction of the radioactivity in the [32P]ADP-ribosylated RT6.1. The reduced radioactivity was recovered from the incubation medium as [32P]ADP-ribose. This reduction was effectively inhibited by the addition of ADP-ribose to the reaction mixture. Moreover, readdition of NAD+ caused the ADP-ribosylation of RT6.1 again. Thus, the ADP-ribosylation of RT6.1 appeared to proceed reversibly in intact rat lymphocytes.

Accumulation of cyclic ADP-ribose measured by a specific radioimmunoassay in differentiated human leukemic HL-60 cells with all-trans-retinoic acid

Cyclic adenosine diphosphoribose (cADPR) is a novel candidate for the mediator of Ca2+ release from intracellular Ca2+ stores. The formation of this cyclic nucleotide is catalyzed by not only Aplysia ADP-ribosyl cyclase but also an ecto-form enzyme of NAD+ glycohydrolase (NADase), which was previously identified as all-trans-retinoic acid (RA)-inducible CD38 in human leukemic HL-60 cells. In the present study, we developed a radioimmunoassay specific for cADPR, by which more than 100 fmol of cADPR could be detected without any interference by other nucleotides. The possible involvement of CD38 in the formation of cellular cADPR was investigated with the radioimmunoassay method. A marked increase in cellular cADPR was accompanied by all-trans-RA-induced differentiation of HL-60 cells. Moreover, a high level of cellular cADPR was observed in other leukemic cell lines, in which CD38 mRNA was expressed. Thus, CD38, which was initially identified as an NADase, appeared to be responsible for the formation of cellular cADPR.

Purification and characterization of the G203T mutant alpha i-2 subunit of GTP-binding protein expressed in baculovirus-infected Sf9 cells

We expressed the Gly203-->Thr (G203T) mutant of Gi2alpha, which was expected to show a dominant-negative phenotype in Gi2-mediated signal transduction, in baculovirus-inefected Sf9 cells and purified the mutant alpha subunit for its characterization. The rate of dissociation of GDP from G203T Gi2alpha was 3- to 4-fold faster than that from wild type Gi2alpha, but their kappacat values for GTP hydrolysis were almost the same. The affinities of the two Gi2alpha proteins for the beta gamma subunits of G proteins to form alpha beta gamma timers, which served as substrates for pertussis toxin-catalyzed ADP-ribosylation, were the same. In marked contrast, G203T Gi2alpha was unable to form a tight complex with a non- hydrolyzable analog (GTP[gammaS) of GTP; bound GTP[gammaS] was readily released from the mutant Gi2alpha even in the presence of a high concentration of Mg2+. Its susceptibility to tryptic digestion also revealed that GTP[gammaS]-bound G203T Gi2alpha formed a conformation apparently different from that of the GTP[gammaS]-bound form of wild-type Gi2alpha. Both the G203T and wild-type Gi2alpha proteins were capable of coupling with membrane-bound alpha2-adrenergic receptors, resulting in the formation of receptor-G protein complexes with high affinity for agonists. However, GTP[gammaS]-dependent uncoupling from high-affinity receptors was markedly attenuated in the case of G203T Gi2alpha. Thus, G203T-mutated Gi2alpha had a unique property in terms of coupling to membrane receptors, in addition to the previously expected defect in the active conformation of the GTP-bound form of Gi2alpha.

G beta gamma directly binds to the carboxyl terminus of the G protein-gated muscarinic K+ channel, GIRK1

beta gamma Subunits of heterotrimeric GTP-binding proteins (G beta gamma) activate the inwardly rectifying muscarinic K+ channel, GIRK1. The significant role for the carboxyl (C) terminus of GIRK1 in this interaction has been suggested. However, it is still unknown whether G beta gamma directly interacts with GIRK1. To elucidate the molecular basis of G beta gamma-activation of GIRK1, we examined the binding properties of G beta gamma to the C terminus of GIRK1 cloned from mouse brain cDNA library (MB-GIRK1). The C terminus of MB-GIRK1 fused with glutathione S-transferase directly bound to purified G beta gamma. Incubation of the C terminus with Gi pretreated with GTP gamma S, but not with GDP, resulted in the binding of Gi beta gamma to the protein. Purified G alpha-GDP, but not G alpha-GTP gamma S, inhibited the binding of G beta gamma to the fusion protein. These results indicate that G beta gamma dissociated from G alpha may directly bind to the C terminus of GIRK1.

Stimulation of mouse DNA primase-catalyzed oligoribonucleotide synthesis by mouse DNA helicase B

Many prokaryotic and viral DNA helicases involved in DNA replication stimulate their cognate DNA primase activity. To assess the stimulation of DNA primase activity by mammalian DNA helicases, we analyzed the synthesis of oligoribonucleotides by mouse DNA polymerase alpha-primase complex on single-stranded circular M13 DNA in the presence of mouse DNA helicase B. DNA helicase B was purified by sequential chromatography through eight columns. When the purified DNA helicase B was applied to a Mono Q column, the stimulatory activity for DNA primase-catalyzed oligoribonucleotide synthesis and DNA helicase and DNA-dependent ATPase activities of DNA helicase B were co-eluted from the column. The synthesis of oligoribonucleotides 5-10 nt in length was markedly stimulated by DNA helicase B. The synthesis of longer species of oligoribonucleotides, which were synthesized at a low level in the absence of DNA helicase B, was inhibited by DNA helicase B. The stimulatory effect of DNA helicase B was marked at low template concentrations and little or no effect was observed at high concentrations. The mouse single-stranded DNA binding protein, replication protein A (RP-A), inhibited the primase activity of the DNA polymerase alpha-primase complex and DNA helicase B partially reversed the inhibition caused by RP-A.

A periodic network of G protein beta gamma subunit coexisting with cytokeratin filament in starfish oocytes

Heterotrimeric G proteins are membrane-bound and carry signals from activated receptors on plasma membranes to cytoplasmic effector enzymes and channels. In starfish oocytes, the beta gamma subunit of G protein mediates 1-methyladenine stimulation of oocyte maturation. In order to investigate the localization of beta gamma subunits in starfish oocytes during oocyte maturation, we raised a monoclonal antibody against the beta subunit. By immunofluorescence microscopy using the antibody, immature oocytes show a network of fibers in the cytoplasm. The staining of fibers is beaded with a periodicity of 0.7 microns. The same staining pattern is obtained by anti-gamma subunit antibody. In addition, the fibers are stained by anti-cytokeratin antibody. These results indicate that the G protein beta gamma subunit coexists with cytokeratin filaments in starfish oocytes. Stimulation of oocyte maturation by 1-methyladenine causes the beta gamma subunit to be disassembled.

Antiallergic constituents from oolong tea stem

The antiallergic constituents of oolong tea stem were examined. The stem extracts inhibited the 48 h homologous passive cutaneous anaphylaxis (PCA) reactions or rats in a dose-dependent manner and showed the same extent of inhibitory activity as ketotifen. All antiallergic constituents from the stem were concentrated into chloroform and ethyl acetate fractions, when extracted by various solvents. These fractions were treated with polyvinylpolypyrrolidone (PVPP), which resulted in the elimination of antiallergic activity in the ethyl acetate fraction, suggesting that one of the antiallergic constituents may be tea catechins. Then, six kinds of catechins, (-)-epigallocatechin gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), -epicatechin (EC), (+)-catechin (C) and (-)-gallocatechin gallate (GCG), were isolated from the ethyl acetate fraction, and the inhibitory activity of these catechins on histamine release from rat peritoneal mast cells passively sensitized with anti-egg albumin (EA) IgE antibody was investigated. Among these catechins, significant inhibitory activity was observed in all the catechins except for EC. In addition, the inhibitory activity of GCG was greater than that of EGCG, which is well known to be an antiallergic constituent in tea. These results suggest that GCG may be a novel antiallergic constituent among tea catechins, and also the most potent.

Mapping of G protein coupling sites of the angiotensin II type 1 receptor

Angiotensin II type 1 (AT1) receptors have been identified in a wide variety of tissues, including the kidney, liver, adrenal gland, cardiovascular system, and brain. AT1 receptors also mediate complex signaling mechanisms that elicit a diversity of specific physiological effects. The rat AT1A receptor has seven transmembrane domains and couples with three distinct G proteins: Gq, Gi, and Go. But it is unknown which domains of AT1A couple with and activate each type of G protein. To identify the domains responsible for the activation of various types of G protein, we studied the effect of five different synthetic peptides representing different domains of cytosolic segments of the rat AT1A receptor on the binding of the 35S-labeled stable analogue of GTP, GTP gamma S. Peptides P-3, which is located in the N-terminal region of the putative third intracellular loop of AT1A (residues 216 through 230), and P-5 (residues 306 through 320), corresponding to the N-terminal region of the C-terminal tail, were found to activate purified Gi1, Gi2, and Go proteins. These results indicate that not only the third cytosolic loop but also the C-terminal cytosolic domain of AT1A is important for Gi1, Gi2, and Go protein coupling and activation.

Potentiation of Gi-mediated phospholipase C activation by retinoic acid in HL-60 cells. Possible role of G gamma 2

Differentiated HL-60 cells acquire responsiveness to fMet-Leu-Phe (fMLP), which activates phospholipase C and O2- generation in a pertussis toxin-sensitive manner. Addition of retinoic acid (RA) for the last 24 h during dimethyl sulfoxide (Me2SO)-induced differentiation enhanced fMLP-dependent signals and interaction between fMLP receptor and G(i). RA modifies both the function and subunit composition of G(i)2, the predominant G(i) of HL-60 membranes, as shown by comparing purified G(i)2 from membranes of Me2SO-treated cells (D-G(i)2) to G(i)2 from membranes of cells treated with both Me2SO and RA (DR-G(i)2). As compared to D-G(i)2, DR-G(i)2 induced more fMLP binding when added to membranes of pertussis toxin-treated HL-60 cells and, in the presence of GTP gamma S, stimulated beta gamma-sensitive phospholipase C in extracts of HL-60 cells to a much greater extent at a lower concentrations. Immunoblasts revealed that RA induced expression of the gamma 2 subunit, which was otherwise undetectable in G(i)2 purified from HL-60 cells or in HL-60 membranes. Possibly by inducing expression of gamma 2, RA alters two functions of the G(i) beta gamma subunit, modulation of fMLP receptor-G(i)2 coupling and activation of the effector, Phospholipase C.

A subtype of opioid kappa-receptor is coupled to inhibition of Gi1-mediated phospholipase C activity in the guinea pig cerebellum

PLC activity was stimulated either by 1-100 microM of GTP or by 100-3,000 microM Ca2+ in lysed synaptosomal membranes of the guinea pig cerebellum. The kappa-opioid receptor agonist selectively inhibited the PLC activity stimulated by 100 microM GTP, but not by 100-3,000 microM Ca2+. Pretreatment of membranes with PTX abolished such a kappa-agonist-induced inhibition of PLC activity. The reconstitution of Gi1, but not of Go purified from porcine brains with PTX-treated membranes showed a complete recovery of the kappa-agonist-inhibition of PLC activity. These findings suggest that a novel subtype kappa-receptor mediates inhibition of PLC through inhibiting the intrinsic activity of PTX-substrate G-proteins.

Enzyme properties of Aplysia ADP-ribosyl cyclase: comparison with NAD glycohydrolase of CD38 antigen

An ecto-enzyme of NAD glycohydrolase (NADase) induced by retinoic acid in HL-60 cells is attributed to the molecule of CD38 antigen [Kontani, K., Nishina, H., Ohoka, Y., Takahashi, K., and Katada, T. (1993) J. Biol. Chem. 268, 16895-16898]. CD38 antigen has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase which generates cyclic adenosine diphosphoribose (cADPR) and nicotinamide (NA) from beta-NAD+. On the basis of this sequence homology, we compared enzyme properties between CD38 NADase expressed as a fusion protein in Escherichia coli and ADP-ribosyl cyclase purified from the ovotestis of Aplysia kurodai. 1) beta-NAD+ analogs, nicotinamide 1, N6-ethenoadenine dinucleotide, and nicotinamide hypoxanthine dinucleotide, did not serve as good substrates for the ADP-ribosyl cyclase, suggesting that the intact adenine ring of beta-NAD+ was required for the cyclase-catalyzed reaction. On the other hand, CD38 NADase utilized the NAD analogs to form ADP-ribose and NA. 2) Kinetic analyses of the ADP-ribosyl cyclase reaction revealed that NA was first released from the substrate (beta-NAD+)-enzyme complex, followed by the release of another product, cADPR, which was capable of interacting with the free enzyme. 3) The enzyme reaction catalyzed by the ADP-ribosyl cyclase was fully reversible; beta-NAD+ could be formed from cADPR and NA with a velocity similar to that observed in the degradation of beta-NAD+. However, CD38 NADase did not catalyze the reverse reaction to form beta-NAD+ from ADP-ribopase and NA. 4) The CD38 NADase activity was, but the ADP-ribosyl cyclase activity was not, inhibited by dithiothreitol.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of pertussis toxin-sensitive mechanism in retinoic acid-induced differentiation of human leukemic HL-60 cells

Retinoic acid-induced differentiation of human leukemic HL-60 cells is accompanied with the early induction of an ecto-enzyme of NAD+ glycohydrolase (NADase), which has recently been identified as human leukocyte cell surface antigen CD38 [Kontani, K. et al. (1993) J. Biol. Chem. 268, 16895-16898]. The terminal cell differentiation attendant upon the cell growth arrest was, but the early induction of CD38 NADase activity was not, inhibited by prior treatment of HL-60 cells with pertussis toxin, which catalyzed ADP-ribosylation of the membrane-bound alpha beta gamma-trimeric GTP-binding proteins. The prior treatment was, however, not essential for the toxin-induced inhibition of the cell differentiation; the inhibition by the addition of pertussis toxin was still observed even after retinoic acid-induced expression of CD38 antigen. This suggested that a pertussis toxin-sensitive mechanism was involved in a late process of cell differentiation. Indeed, HL-60 cells appeared to secrete a differentiation-supporting factor in response to retinoic acid, since the cell differentiation was accelerated and potentiated upon culture of the cells in a conditioned medium prepared from retinoic acid-treated cells. The action of the differentiation-supporting factor was destroyed by heating and markedly attenuated in pertussis toxin-pretreated HL-60 cells. Thus, the whole process of the retinoic acid-induced cell differentiation appeared to consist of two distinguishable periods in terms of sensitivity to pertussis toxin; the toxin-insensitive early period characterized by the induction of CD38 NADase activity and the toxin-sensitive late period in which the secretion of a differentiation-supporting factor might be involved.

ADP-ribosylarginine glycohydrolase catalyzing the release of ADP-ribose from the cholera toxin-modified alpha-subunits of GTP-binding proteins

A rat glycohydrolase which catalyzes the hydrolysis of ADP-ribosylarginine was expressed in Escherichia coli and purified to homogeneity for characterization of its enzymatic properties. The purified glycohydrolase catalyzed the hydrolysis of N-glycoside linked ADP-ribosylarginine on the alpha-subunits of stimulatory GTP-binding proteins (Gs) and cholera toxin A1-subunit that had been modified by cholera toxin and NAD. Nonmuscle actin of which an arginine residue was ADP-ribosylated by botulinum C2 toxin also served as a substrate of the glycohydrolase. On the other hand, the glycohydrolase did not hydrolyze ADP-ribosylated cysteine on the alpha-subunits of pertussis toxin-substrate GTP-binding proteins, ADP-ribosylated diphthamide on elongation factor 2, or ADP-ribosylated asparagine on rho GTP-binding proteins. The rate of the reaction catalyzed by the glycohydrolase was affected by nucleotide-binding form of the ADP-ribosylated substrate proteins; the GDP-bound form of the modified Gs-alpha was more rapidly hydrolyzed than the guanosine 5'-(3-O-thio)triphosphate-bound form. Interestingly, the glycohydrolase activity was markedly inhibited by mM order concentration of ATP in addition to ADP-ribose, the product of the enzyme reaction, though ADP had no inhibitory effect on the activity. Moreover, alpha NAD, but not beta NAD, inhibited the enzyme activity, suggesting that the glycohydrolase reaction was stereospecific for the alpha-anomer.

Cell surface antigen CD38 identified as ecto-enzyme of NAD glycohydrolase has hyaluronate-binding activity

An ecto-enzyme of NAD glycohydrolase induced by retinoic acid in human leukemic HL-60 cells is attributed to the molecule of leukocyte cell surface antigen CD38 (Kontani, K., et al. (1993) J. Biol. Chem. 268, 16895-16898). The cell surface antigen has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase that catalyzes the conversion of NAD to cyclic ADP-ribose with a calcium-mobilizing activity. A putative hyaluronate (HA)-binding motif which has recently been identified in CD44 antigen existed in the extracellular domain and intracellular amino terminus of CD38 antigen. CD38 antigen was indeed capable of binding to HA in a manner dependent on ionic strength. By contrast, no binding activity was found in Aplysia ADP-ribosyl cyclase. Thus CD38 antigen, like CD44 antigen characterized as a HA-receptor (or binding) protein, may function as an adhesion molecule.

Characterization of botulinum C3-catalyzed ADP-ribosylation of rho proteins and identification of mammalian C3-like ADP-ribosyltransferase

The exoenzyme C3 produced by Clostridium botulinum catalyzes ADP-ribosylation of rho gene products which belong to a family of small molecular-weight GTP-binding proteins. The C3 enzyme-catalyzed ADP-ribosylation of rho proteins partially purified from bovine brain was markedly activated by certain types of detergents or phospholipids and by endogenous factors present in the brain cytosol. Rho A protein that had been expressed in E. coli and subsequential purified was readily ADP-ribosylated by the C3 enzyme even in the absence of the activating factors. These results suggest that partially purified rho proteins contain an inhibitor, probably rho GDI (GDP-dissociation inhibitor for rho p21), of C3-catalyzed ADP-ribosylation. The activity of an endogenous enzyme, having the same substrate as botulinum C3 enzyme, was also found in brain cytosol. The enzyme activity was partially purified and characterized. The enzyme appeared to have a molecular mass of approximately 20,000 on a gel filtration and displayed unique properties similar to those observed with the botulinum C3 enzyme. The alpha-subunits of alpha beta gamma-trimeric G proteins which served as the substrates of cholera or pertussis toxin were not ADP-ribosylated by the brain enzyme.

Differential activation of adenylyl cyclase by protein kinase C isoenzymes

Cyclic AMP production within cells is altered upon protein kinase C (PKC) activation; however, whether PKC directly modulates adenylyl cyclase (AC) catalytic activity has been controversial. Molecular studies have elucidated the existence of multiple PKC isoenzymes although the functional role of this diversity is not clear. Using purified PKC and AC isoenzymes, we demonstrate that PKC zeta directly phosphorylates type VAC, leading to an approximate 20-fold increase in its catalytic activity, a significantly larger enhancement than that achieved with forskolin (approximately 5-fold), the most potent activator of AC. When forskolin and PKC phosphorylation are combined, type V AC catalytic activity is increased 100-fold over basal levels. The two PKC isoenzymes (alpha and zeta) are additive in their capacity to activate AC, although PKC alpha is less potent than PKC zeta. Our data indicate that PKC can directly and potently regulate AC activity in an isoenzyme-specific manner, suggesting that direct cross-talk plays a major role in coordinating the activity of these two principal signal transduction pathways.

[Structure and function of heterotrimeric G-proteins]

In mammals, G-protein alpha, beta, gamma polypeptides are encoded by at least 16, 4 and 7 genes, respectively. G alpha-subunits bind and hydrolyze GTP and have the sites for bacterial toxin-catalyzed ADP-ribosylation. A structural model of G alpha-subunits can be defined on the basis of similarities between G alpha and other members of the GTP-binding proteins. The resulting G alpha model specifies the spatial relationship among the guanine nucleotide binding site, the binding site of the G beta gamma-subunit complex, likely regions of effector and receptor interaction, and sites of cholera or pertussis toxin-induced modification. The architecture of the G alpha core is the same as that of p21ras. Experimental evidence from immunological, molecular genetic and biochemical studies support the G alpha model. The G alpha-subunits alone were previously thought to act on the effector enzymes; However, recent evidence indicates that the G beta gamma-dimer also plays an important part in effector activation.

An intrinsic guanine nucleotide exchange inhibitor in Gi2 alpha. Significance of G-protein self-suppression which antagonizes receptor signal

The alpha subunit of Gi2 (Gi2 alpha) is a member of the heterotrimeric G protein family, which transduces receptor signals as a proto-oncogene product. We have found a novel self-suppressive region in Gi2 alpha near its C terminus. A polypeptide consisting of residues 338-352 of Gi2 alpha (Gi2 alpha-339-352) antagonizes receptor- and receptor peptide-stimulated Gi2 alpha activation, without affecting basal activity. Antagonism by Gi2 alpha-338-352 is attributable to an interaction with activated Gi2 alpha, which is not competitive with receptor polypeptides. Combined with the reports suggesting the presence of self-suppressive domains in a juxta-C-terminal portion of Gi2 alpha and G(o) alpha, this study supports the hypothesis that Gi2 alpha-338-352 constitutes an intrinsic guanine nucleotide exchange inhibitor, which in turn antagonizes receptor stimulation, suggesting that G proteins are activated by receptors through relaxation of a self-suppressive conformation.