The effect of iloprost on the ADP-ribosylation of Gs alpha (the alpha-subunit of Gs)

Modification of adipocyte membrane adenylyl cyclase activity by NAD: evidence against NAD-induced endogenous ADP-ribosylation of Gsalpha protein

Treatment of saponin-permeabilized adipocytes with NAD enhanced adenylyl cyclase activity stimulated by GTgammaS, [Al/F(4)](-), isoproterenol, and forskolin in membrane fractions and potentiated isoproterenol-induced cAMP accumulation in whole cells. In parallel, when permeabilized adipocytes were incubated with [(32)P]NAD, there was significant incorporation of [(32)P]ADP-ribose in a 44-kDa acceptor membrane protein. This reaction was inhibited by l-arginine and was enhanced by the addition of GTPgammaS. Surprisingly, this 44-kDa protein could not be identified as Gs protein: (1) It was not recognized by Gsalpha specific antibody; (2) it did not comigrate with the major cholera toxin substrates in either 10% SDS-PAGE or two-dimensional electrophoresis; (3) a pretreatment of adipocytes with NAD did not decrease cholera toxin-mediated ADP-ribosylation of Gsalpha proteins on membrane fractions. Our results indicate that NAD did not induce endogenous ADP-ribosylation of Gsalpha in permeabilized rat adipocytes but nonetheless modified the adenylyl cyclase response.

Inhibition of cADP-ribose formation produces vasodilation in bovine coronary arteries

cADP-ribose (cADPR) induces the release of Ca(2+) from the intracellular stores of coronary artery smooth muscle cells. However, little is known about the role of cADPR-mediated intracellular Ca(2+) release in the control of vascular tone. The present study examined the effects of nicotinamide, a specific inhibitor of ADP-ribosylcyclase, on the vascular tone of bovine coronary arteries. A bovine coronary artery homogenate stimulated the conversion of nicotinamide guanine dinucleotide into cGDP-ribose, which is a measure of ADP-ribosylcyclase activity. Nicotinamide significantly inhibited the formation of cGDP-ribose in a concentration-dependent manner: at a concentration of 10 mmol/L, it reduced the conversion rate from 3.34+/-0.11 nmol. min(-1). mg(-1) of protein in control cells to 1.42+/-0.11 nmol. min(-1). mg(-1) of protein in treated cells, a 58% reduction. In U46619-precontracted coronary artery rings, nicotinamide produced concentration-dependent relaxation. Complete relaxation with nicotinamide occurred at a dose of 8 mmol/L; the median inhibitory concentration (IC(50)) was 1.7 mmol/L. In the presence of a cell membrane-permeant cADPR antagonist, 8-bromo-cADPR, nicotinamide-induced vasorelaxation was markedly attenuated. Pretreatment of the arterial rings with ryanodine (50 micromol/L) significantly blunted the vasorelaxation response to nicotinamide. However, iloprost- and adenosine-induced vasorelaxation was not altered by 8-bromo-cADPR. Moreover, nicotinamide significantly attenuated KCl- or Bay K8644-induced vasoconstriction by 60% and 70%, respectively. These results suggest that the inhibition of cADPR formation by nicotinamide produces vasorelaxation and blunts KCl- and Bay K8644-induced vasoconstriction in coronary arteries and that the cADPR-mediated Ca(2+) signaling pathway plays a role in the control of vascular tone in coronary circulation.

11,12-Epoxyeicosatrienoic acid stimulates endogenous mono-ADP-ribosylation in bovine coronary arterial smooth muscle

The role of endogenous ADP-ribosylation in mediating the activation of the Ca(2+)-activated K(+) channels was determined in bovine coronary arteries. Endogenous ADP-ribosylation was examined by incubating coronary arterial homogenates or lysates of cultured coronary arterial smooth muscle cells with [adenylate-(32)P]NAD. Four (32)P-labeled proteins were observed at 51, 52, 80, and 124 kDa in the homogenates and lysates. This reaction was enhanced by the addition of 11,12-epoxyeicosatrienoic acid (11,12-EET), a cytochrome P450-derived eicosanoid, and GTP to the incubation. By Western blot analysis, 42- and 70-kDa proteins were recognized by specific antibodies against ADP-ribosyltransferase in the coronary arterial homogenates and smooth muscle cell lysate but not in the lysate of endothelial cells. The 52-kDa acceptor protein of endogenous ADP-ribosylation comigrated with a protein ADP-ribosylated by cholera toxin and was recognized and immunoprecipitated by an anti-G(S)alpha antibody. These results suggest that G(S)alpha is one of several acceptors of the ADP-ribose moiety. As shown by the patch-clamp technique, 11,12-EET stimulated the activation of the K(+) channels in the smooth muscle cells, and this activation was completely blocked by novobiocin, vitamin K(1), 3-aminobenzamide, and m-iodobenzylguanidine, inhibitors of endogenous mono-ADP-ribosyltransferases. We conclude that endogenous mono-ADP-ribosyltransferases are present in smooth muscle from bovine coronary arteries. These enzymes transfer ADP-ribose to the cellular proteins such as G(S)alpha and may mediate intracellular signal transduction in coronary vascular smooth muscle. In the coronary circulation, the ADP-ribosylation signaling pathway may play an important role in mediating the activation of the K(+) channels induced by 11,12-EET.

Characterization of glycosylphosphatidylinositiol-anchored, secreted, and intracellular vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation is a posttranslational modification of proteins in which the ADP-ribose moiety of nicotinamide adenine dinucleotide is transferred to an acceptor amino acid. Five mammalian ADP-ribosyltransferases (ART1--ART5) have been cloned and expression is restricted to tissues such as cardiac and skeletal muscle, leukocytes, brain, and testis. ART1 and ART2 are glycosylphosphatidylinositol (GPI)-anchored ectoenzymes. ART5 appears not to be GPI-linked and may be secreted. In skeletal muscle and lymphocytes, ART1 modifies specific members of the integrin family of adhesion molecules, suggesting that ADP-ribosylation affects cell-matrix or cell-cell interactions. In lymphocytes, ADP-ribosylation of surface proteins is associated with changes in p56lck tyrosine kinase-mediated signaling. The catalytic sites of bacterial toxins and vertebrate transferases have conserved structural features, consistent with a common reaction mechanism. ADP-ribosylation can be reversed by ADP-ribosylarginine hydrolases, resulting in the regeneration of free arginine. Thus, an ADP-ribosylation cycle may play a regulatory role in vertebrate tissues.

Characterization of alpha(s)-immunoreactive ADP-ribosylated proteins in postmortem human brain

ADP-ribosylation of the stimulatory G protein alpha subunit, alpha(s), has been demonstrated in a number of different mammalian tissues. However, little is known about the occurrence and role of this process in modifying alpha(s) levels/function in human brain. In the present study, endogenous and cholera toxin (CTX)-catalyzed [32P]ADP-ribosylated products were characterized in postmortem human temporal cortex by (1) immunoprecipitation with alpha(s) antisera (RM/1), (2) comparisons of immunoblots and autoradiograms of the [32P]ADP-ribosylated products, and (3) limited protease digestion. Of the three major endogenous [32P]ADP-ribosylated products (48, 45, and 39 kDa) in postmortem brain, the 48-kDa and 45-kDa bands were clearly identified as alpha(s-L) (long isoform) and alpha(s-S) (short isoform), respectively. RM/1 immunoprecipitated the 39-kDa [32P]ADP-ribosylated protein, and overlays of immunoblots and autoradiograms showed that this product corresponded to an alpha(s)-like-immunoreactive protein. Furthermore, limited protease digestion of the 39-kDa endogenous [32P]ADP-ribosylated band generated peptide fragments similar to both endogenous and CTX-catalyzed [32P]ADP-ribosylated alpha(s-S). Two major CTX-catalyzed [32P]ADP-ribosylated products were also identified as alpha(s-L) (52 kDa) and alpha(s-S) (45 kDa). These findings clearly demonstrate that alpha(s) is a substrate for endogenous and CTX-catalyzed [32P]ADP-ribosylation in postmortem human brain. Furthermore, a lower molecular weight alpha(s)-like immunoreactive protein is also expressed in human brain and is a substrate for endogenous but not CTX-catalyzed [32P]ADP-ribosylation.

Selective formation of Gsalpha-MHC I complexes after desensitization of human platelets with iloprost

Prolonged treatment of human platelets with the adenylate cyclase-stimulating prostacyclin analog iloprost leads to reduction in cAMP formation. Previous studies have demonstrated that this may be ascribed to modification of both receptor and Gsalpha function rather than of the catalytic component of adenylate cyclase [Mollner, S., Deppisch, H. & Pfeuffer, T. (1992) Eur. J. Biochem. 210, 539-544]. Iloprost-induced desensitization was accompanied by the formation of a Gsalpha-containing 90-kDa product in membranes treated with the bifunctional cross-linker 1,6-bismaleimidohexane. The cAMP-inducing prostanoid PGD2, which does not promote desensitization, did not cause formation of the 90-kDa species either. The long-term effect of the common G-protein activator [AlF4]- on human platelet adenylate cyclase was shown in many respects to be comparable with that of iloprost. However, [AlF4]- treatment also failed to induce the 90-kDa species, showing that different mechanisms of desensitization were operating. Treatment of the cross-linked 90-kDa complex with PNGase F demonstrated the glycoprotein nature of the Gsalpha-associated component. The 90-kDa cross-linked product was purified by consecutive immunoaffinity chromatography and preparative PAGE to apparent homogeneity. Analysis of the purified protein by MS suggested that, besides Gsalpha, the heavy chain of MHC I (HLA-A2) was part of the complex. This was confirmed by coprecipitation of Gsalpha by the monoclonal anti-(MHC I) antibody W6/32.

Analysis of GTP-binding proteins, phosphoproteins, and cytosolic calcium in functional heterogeneous human blood platelet subpopulations

The biochemical basis for the functional heterogeneity of human blood platelets was investigated in terms of protein phosphorylation, cytoplasmic calcium ([Ca2+]i), the ratio of 46 and 50 kDa vasodilator-stimulated protein (VASP), and GTP-binding proteins (G-proteins). Platelets were fractionated by density. Comparing resting low-density platelets (LDP) to high-density platelets (HDP) revealed higher phosphorylation of proteins in the 47, 31, and 24 kDa ranges. A higher phosphorylation of the 20 kDa protein in LDP compared to HDP was related to an enhanced [Ca2+]i, an increased ADP-ribosylation of the inhibitory G-protein (G(i alpha1-3)) and rhoA, and a decreased ADP-ribosylation of the stimulatory G-protein (G(s alpha)). The differences in the ribosylation patterns of the subpopulations were not influenced by thrombin stimulation or exposure to prostaglandin E1 (PGE1). An 18 kDa phosphoprotein was more highly phosphorylated in resting HDP than in LDP. Thrombin exposure caused dephosphorylation of the 18 kDa phosphoprotein in the HDP, but generally increased phosphorylation of both HDP and LDP in the 47, 31, 24, and 20 kDa bands. Preincubation with prostaglandin E1 or sodium nitroprusside diminished the subsequent thrombin-induced increase in phosphorylation, particularly in HDP. In unstimulated HDP, the 50 kDa VASP phospho form was enhanced, whereas in unstimulated LDP the 46 kDa VASP dephospho form was increased. Our findings suggest that the functional heterogeneity of platelets is partly derived from differences in signal transduction mechanisms reflected in varying phosphoprotein patterns and G-protein properties of platelet stimulatory and inhibitory pathways.

Cell-surface ADP-ribosylation of fibroblast growth factor-2 by an arginine-specific ADP-ribosyltransferase

Basic fibroblast growth factor (FGF-2) appeared to be ADP-ribosylated on the surface of adult bovine aortic arch endothelial and human hepatoma cells. Further characterization of this reaction with cells expressing an arginine-specific, glycosylphosphatidylinositol-anchored, mono-ADP-ribosyltransferase demonstrated that FGF-2 is ADP-ribosylated on arginine. Incubation of transformed cells with FGF-2 and [adenylate-32P]nicotinamide-adenine dinucleotide (NAD) resulted in the rapid incorporation of [32P]ADP-ribose into FGF-2 in a time- and concentration-dependent manner, with labelling averaging 3 mol of ADP-ribose/mol of FGF-2. Excess ADP-ribose had no effect on these reactions, whereas excess NAD inhibited the ADP-ribosylation of FGF-2, consistent with an enzymic rather than a non-enzymic ADP-ribosylation reaction. Heparin also inhibited the ADP-ribosylation reaction, whereas a neutralizing polyclonal anti-peptide antibody had no effect. Furthermore, the addition of putative receptor binding domain peptide analogues of FGF-2 reduced the maximal ADP-ribosylation of FGF-2. These results identify the cell-surface ADP-ribosylation of FGF-2 as a potentially ubiquitous event.

Mono-ADP-ribosylation: a reversible posttranslational modification of proteins

Mono-ADP-ribosyltransferase activity has been detected in numerous vertebrate tissues and transferase cDNAs from a few species have recently been cloned. In vitro ADP-ribosylation has been demonstrated with diverse substrates such as phosphorylase kinase, actin, and Gs alpha resulting in the alteration of substrate function. ADP-ribosylation of endogenous target proteins has been observed in chicken heterophils, rat brain, and human platelets, and integrin alpha 7 was found to be the endogenous substrate of the GPI-anchored rabbit skeletal muscle transferase. The reversibility of ADP-ribosylation is made possible by ADP-ribosylarginine hydrolases which have been isolated and cloned from rodent and human tissues. The transferases and hydrolases could in principle form an intracellular ADP-ribosylation regulatory cycle. In the case of the skeletal muscle transferases, however, processing of ADP-ribosylated integrin alpha 7 is carried out by phosphodiesterases and possibly phosphatases (Fig. 1). Most bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 family of proteins, share a common catalytic-site structure despite a lack of overall sequence identity. The transferases that have been studied thus far possess a critical glutamic acid and other amino acids at the catalytic cleft which function to position NAD for nucleophilic attack at the N-glycosidic linkage for either ADP-ribose transfer or NAD hydrolysis. The amino acid differences among transferases at the active site may reflect different catalytic mechanisms of ADP-ribosylation or may be required for accommodating the different ADP-ribose acceptor molecules.

Structure and function of eukaryotic mono-ADP-ribosyltransferases

ADP-ribosylation of proteins has been observed in numerous animal tissues including chicken heterophils, rat brain, human platelets, and mouse skeletal muscle. ADP-ribosylation in these tissues is thought to modulate critical cellular functions such as muscle cell development, actin polymerization, and cytotoxic T lymphocyte proliferation. Specific substrates of the ADP-ribosyltransferases have been identified; the skeletal muscle transferase ADP-ribosylates integrin alpha 7 whereas the chicken heterophil enzyme modifies the heterophil granule protein p33 and the CTL enzyme ADP-ribosylates the membrane-associated protein p40. Transferase sequence has been determined which should assist in elucidating the role of ADP-ribosylation in cells. There is sequence similarity among the vertebrate transferases and the rodent RT6 alloantigens. The RT6 family of proteins are NAD glycohydrolases that have been shown to possess auto-ADP-ribosyltransferase activity whereas the mouse Rt6-1 is also capable of ADP-ribosylating histone. Absence of RT6+ T cells has been associated with the development of an autoimmune-mediated diabetes in rodents. Humans have an RT6 pseudogene and do not express RT6 proteins. The reversal of ADP-ribosylation is catalyzed by ADP-ribosylarginine hydrolases, which have been purified and cloned from rodent and human tissues. In principle, the transferases and hydrolases could form an intracellular ADP-ribosylation regulatory cycle. In skeletal muscle and lymphocytes, however, the transferases and their substrates are extracellular membrane proteins whereas the hydrolases described thus far are cytoplasmic. In cultured mouse skeletal muscle cells, processing of the ADP-ribosylated integrin alpha 7 was carried out by phosphodiesterases and possibly phosphatases, leaving a residual ribose attached to the (arginine)protein. Several bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 alloantigens share regions of amino acid sequence similarity, which form, in part, the catalytic site. The catalytic cleft, found in the bacterial toxins that have been studied thus far, contains a critical glutamate and other amino acids that function to position NAD for nucleophilic attack at the N-glycosidic linkage, for either ADP-ribose transfer or NAD hydrolysis. Amino acid differences among the transferases at the active site may be required for accommodating the different ADP-ribose acceptor molecules.

Identification of gangliosides as inhibitors of ADP-ribosyltransferases of pertussis toxin and exoenzyme C3 from Clostridium botulinum

We have previously reported the presence of an endogenous inhibitory activity in bovine brain for the ADP-ribosylation of GTP-binding proteins catalyzed by pertussis toxin (PT) (Hara-Yokoyama, M., and Furuyama, S. (1989) Biochem. Biophys. Res. Commun. 160, 67-71). In the present study, we identified the inhibitor as a ganglioside. The screening of various gangliosides revealed that GQ1b alpha most effectively inhibited the ADP-ribosyltransferase activities of both the holoenzyme and the catalytic subunit of PT. GQ1b alpha is a ganglioside newly identified as one of the antigens recognized by the cholinergic neuron-specific antibody, anti-Chol-1 alpha (Hirabayashi, Y., Nakao, T., Irie, F., Whittaker, V.P., Kon, K., and Ando, S. (1992) J. Biol. Chem. 267, 12973-12978). GQ1b alpha also inhibited the PT-catalyzed NAD+ glycohydrolysis. Unlike PT activity, the ADP-ribosylation and the NAD+ glycohydrolysis catalyzed by the C3 exoenzyme from Clostridium botulinum type C were inhibited by GT1b and GQ1b. The ADP-ribosylation catalyzed by either PT or the C3 exoenzyme was not inhibited by ceramide, galactocerebroside, or sialic acid. In addition to the inhibitory action of gangliosides on ADP-ribosylation, the importance of gangliosides as regulators of NAD+ metabolism is discussed.

Inhibition of ADP-ribosyltransferase increases synthesis of Gs alpha in neuroblastoma x glioma hybrid cells and reverses iloprost-dependent heterologous loss of fluoride-sensitive adenylate cyclase

Exposure of NG108-15 cells to 50 mM nicotinamide [an inhibitor of mono(ADP-ribosyl)transferase] for 18 hr led to an increase in membrane associated Gs alpha measured either as cholera toxin substrate or by immunoblotting with a specific antiserum. Prolonged exposure of NG108-15 cells to iloprost is followed by homologous loss of iloprost sensitivity, and heterologous loss of fluoride-dependent activation of adenylate cyclase. Nicotinamide reversed the loss of fluoride sensitivity, but failed to restore iloprost-dependent activation of adenylate cyclase. These results with nicotinamide in NG108-15 cells contrasted with those from platelets, which also exhibit heterologous desensitization of fluoride sensitivity following prolonged exposure to iloprost. Treatment of platelets with 50 mM nicotinamide for 18 hr led to an increase of 75.0 +/- 19.4% in the amount of membrane associated cholera toxin substrate. However, there was no associated increase in the abundance of Gs alpha as determined by immunoblotting. Furthermore, in platelets there was no restoration by nicotinamide of the iloprost-dependent loss of fluoride-sensitive adenylate cyclase activity. It follows that heterologous desensitization in platelets is accompanied by inactivation of Gs alpha, which is retained within the plasma membrane in its inactive state. The nicotinamide-dependent increase in the abundance of membrane associated cholera toxin substrate and immunoreactive Gs alpha in NG108-15 cells is associated with an increase of 72.0 +/- 20.3% in the levels of mRNA encoding Gs alpha. The capacity of nicotinamide to increase the abundance of membrane associated Gs alpha was reversed when the cells were cultured in the presence of 20 micrograms/mL cycloheximide. These results suggest that the ability of nicotinamide to increase the abundance of Gs alpha in NG108-15 cells is mediated by de novo protein synthesis.

Nitric oxide modulation of quantal secretion in chick ciliary ganglia

1. Long-term potentiation of quantal secretion was studied at ciliary ganglion synapses of post-hatched birds following tetanic stimulation of the oculomotor nerve and the effects of nitric oxide (NO) on quantal secretion were determined. 2. Tetanic stimulation of the oculomotor nerve at 30 Hz for 20 s at room temperature increased the amplitude of the excitatory postsynaptic potential (EPSP) by about 100%; 1-2 min after the tetanus the EPSP declined exponentially with a time constant of about 10 min (long-term potentiation; LTP). LTP was due to an increase in the quantal content of the EPSP not to a change in quantal size. 3. A component of LTP was shown to be due to the release of NO in the ganglion, as blocking the synthesis of NO with L-arginine methyl ester decreased the potentiation by 70%. 4. Exogenous application of NO using sodium nitroprusside increased the amplitude of the EPSP by more than 30% due to an increase in the quantal content of the EPSP. 5. Both 8-bromo-cGMP and 8-bromo-cAMP increased the quantal content of the EPSP by more than 44% without changing the quantal size. 6. The results suggest that endogenous NO is involved in either the initiation or maintenance phase of LTP. This may occur through an increase in quantal secretion consequent on the action of an elevated cGMP increasing cAMP.

Nitric oxide release and long term potentiation at synapses in autonomic ganglia

1. Long-term potentiation (LTP) of synaptic transmission in autonomic ganglia is reviewed, together with the possible role of nitric oxide (NO) in this process. 2. Calcium levels in preganglionic nerve terminals are elevated during at least the induction phase of LTP following a tetanus as well as during LTP induced by transmitter substances acting on the nerve terminals. Of the large number of calcium-dependent processes in the nerve terminal that might affect transmitter release, only calcium-calmodulin has been shown to be important in both the induction and maintenance of LTP. 3. The possibility that there is a decrease in the open time of nerve-terminal potassium channels following a tetanus, leading to an increase in duration of the terminal action potential and hence an increase in calcium influx and transmitter release is considered. There is little evidence for such an effect as yet for preganglionic nerve terminals. 4. Phosphorylation of potassium channels by cAMP-dependent protein kinase can lead to their inactivation with consequent action potential broadening in some systems. Exogenous cAMP enhances synaptic efficacy at preganglionic nerve terminals. Whether this occurs through an inactivation of potassium channels is not known. 5. Nitric oxide (NO) synthase is present in both sympathetic ganglia and the ciliary ganglia. NO increases synaptic efficacy in both ganglia. In at least the case of ciliary ganglion this is due to elevation of quantal secretion. 6. NO can in some conditions increase the terminal action potential duration in ciliary ganglia, probably through decrease in the Ic potassium current. There is evidence that this happens through cGMP modulating cAMP phosphodiesterases, thereby affecting cAMP phosphorylation of the Ic channel. 7. Blocking NO synthase markedly decreases LTP following a tetanus in the ciliary ganglion. The possibility is considered that NO is released from the terminal during a tetanus and through altering cAMP phosphorylation of Ic enhances transmitter release.

Vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation appears to be a reversible modification of proteins, which occurs in many eukaryotic and prokaryotic organisms. Multiple forms of arginine-specific ADP-ribosyltransferases have been purified and characterized from avian erythrocytes, chicken polymorphonuclear leukocytes and mammalian skeletal muscle. The avian transferases have similar molecular weights of approximately 28 kDa, but differ in physical, regulatory and kinetic properties and subcellular localization. Recently, a 38-kDa rabbit skeletal muscle ADP-ribosyltransferase was purified and cloned. The deduced amino acid sequence contained hydrophobic amino and carboxy termini, consistent with known signal sequences of glycosylphosphatidylinositol (GPI)-anchored proteins. This arginine-specific transferase was present on the surface of mouse myotubes and of NMU cells transfected with the cDNA and was released with phosphatidylinositol-specific phospholipase C. Arginine-specific ADP-ribosyltransferases thus appear to exhibit considerable diversity in their structure, cellular localization, regulation and physiological role.

Agonist regulation of cellular G protein levels and distribution: mechanisms and functional implications

Exposure of cells to agonists of receptors linked to G proteins can result in downregulation of cellular levels or redistribution of G proteins from membranes to the cytosol. Agonist-induced reductions in G protein levels have been observed for members of each of the Gs, Gi and Gq families of G proteins, are likely to be dependent upon the level of receptor expression, and are generally restricted to the G protein(s) with which the receptor interacts. The mechanisms responsible, reviewed here by Graeme Milligan, vary with cell type and include both second messenger-dependent and -independent enhanced protein degradation. Agonist-induced reduction in cellular G protein levels can provide one mechanism for the development of sustained heterologous desensitization.

The alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein forms high-molecular-mass aggregates, concomitant with iloprost-induced desensitization of human platelet adenylyl cyclase

Prolonged treatment of human platelets with the prostacyclin analog iloprost led to desensitization of the response to various prostaglandin derivatives. However, basal adenylyl cyclase activity and stimulation by agents acting directly via Gs, the stimulatory guanine-nucleotide-binding regulatory protein of adenylyl cyclase, were likewise decreased. Reconstitution of desensitized membranes with purified Gs from turkey erythrocytes indicated no alteration in the catalyst itself. However, the function of Gs (in cholate extracts) appeared to be severely impaired when reconstituted with adenylyl cyclase catalyst. Modification of Gs was also indicated by its altered sedimentation in sucrose density gradients. From Western blots, the alpha subunit of Gs, alpha s, from control platelets sedimented as a 5.6S species, while that from desensitized cells appeared at higher S values (in a polydisperse distribution). Activation by guanosine 5'-[gamma-thio]triphosphate of Gs from control platelets shifted alpha s to 3.5-3.7S, while activation of Gs from desensitized platelets induced such shift only for a minor portion of alpha s. This small fraction alone appeared to be susceptible to ADP-ribosylation by cholera toxin/[32P]NAD. Furthermore, an antibody directed against the C-terminal hexadecapeptide of alpha s precipitated much less alpha s from cholate extracts derived from desensitized platelets. Modification of alpha s during desensitization was also suggested from cross-linking experiments using the homobifunctional agent bismaleimidohexane: alpha s from desensitized platelets formed a single product of 80 kDa, while that from untreated platelets yielded a doublet (100 kDa and 110 kDa).

Gs alpha is a substrate for mono(ADP-ribosyl)transferase of NG108-15 cells. ADP-ribosylation regulates Gs alpha activity and abundance

NG108-15 neuroblastoma x glioma somatic hybrid cells were permeabilized in the presence of [32P]NAD+ and then cultured for 18 h. Resolution of the cell proteins on polyacrylamide gels revealed [32P]ADP-ribosylation of five major protein species with molecular mass values of 52 kDa, 44 kDa, 35 kDa, 30 kDa and 25 kDa. A similar pattern of labelling was also seen when NG108-15 cell membranes were incubated with [32P]NAD+ and hydrolysis of the product revealed mono(ADP-ribosyl)ation. Immunoprecipitation of these products with anti-Gs alpha antiserum revealed a single band identical to cholera toxin substrate. Culture of [32P]NAD(+)-loaded cells for 18 h in the presence of 50 mM-nicotinamide inhibited the eukaryotic mono(ADP-ribosyl)transferase activity. Inhibition of the eukaryotic enzyme was also accompanied by an increase in the abundance of Gs alpha, whether measured by Western blotting with anti-Gs alpha antibody (two separate antisera) or by cholera toxin-dependent [32P]ADP-ribosylation. There was no accompanying change in the abundance of G beta. The increase in Gs alpha abundance in nicotinamide-treated NG108-15 cells was accompanied by a 2-fold increase in basal adenylate cyclase activity (measured in the presence of GTP), and by a smaller but significant increase in iloprost-dependent activation of adenylate cyclase. Receptor number or affinity was not affected by nicotinamide, since this treatment did not alter the binding parameters of [3H]iloprost to NG108-15 cell membranes. Short-term exposure of cells to nicotinamide for 1 h revealed no significant difference in either basal or agonist-stimulated adenylate cyclase activity. These results reveal that mono(ADP-ribosyl)ation of Gs alpha by eukaryotic ADP-ribosyltransferase modifies the abundance and activity of Gs alpha in NG108-15 cells, and hence may play a role in the hormonal regulation of cell function.

Modulation of endogenous ADP-ribosylation in rat brain

Endogenous ADP-ribosylation of proteins was measured in homogenates, membranes, and cytosol from rat brain regions. Several proteins were ADP-ribosylated in homogenates, especially a 49 kDa protein. Sodium nitroprusside, a source of nitric oxide, particularly enhanced the ADP-ribosylation of 47 kDa and 39 kDa proteins. Levels of basal and sodium nitroprusside-stimulated ADP-ribosylated proteins were similar, but not identical, in homogenates from the cerebral cortex, hippocampus, striatum, thalamus and cerebellum. In neonatal cerebral cortex, ADP-ribosylation of an additional 110 kDa protein was detected and this was also enhanced by sodium nitroprusside. ADP-ribosylation of the 110 kDa protein was evident one and two days after birth, but not at five days and later. Each protein demonstrated unique sensitivities to sodium nitroprusside and rates of ADP-ribosylation. Cyclic GMP did not mimic the effects of sodium nitroprusside. Mg2+ inhibited ADP-ribosylation of the 49 kDa and 47 kDa proteins but had a smaller effect on the 39 kDa protein. ADP-ribosylation in the cytosol predominantly affected only a single protein of 39 kDa, and this was stimulated by sodium nitroprusside and by addition of cofactors necessary for activation of nitric oxide synthase. Several proteins in membranes were ADP-ribosylated and the 49 and 47 kDa proteins were released from the membranes coincidentally with ADP-ribosylation. The predominate substrates of endogenous ADP-ribosylation did not appear to be substrates for pertussis toxin-induced ADP-ribosylation. These and previously published results indicate that nitric oxide generated from sodium nitroprusside or endogenous sources may have modulatory effects through regulation of the endogenous ADP-ribosylation of proteins.

Cyclic AMP produces desensitization of prostacyclin and adenosine A2 receptors in hybrid cell lines but does not affect Gs function

Prostacyclin and adenosine A2 receptors stimulate adenylate cyclase activity in the related somatic hybrid cell lines NG108-15 and NCB20. The role of cAMP in the desensitization of these receptors has been examined. Pretreatment for 17 h with forskolin or 8-bromo-cAMP had the same effect in both cell lines. There was no change in the response to sodium fluoride or forskolin, suggesting that the function of Gs and adenylate cyclase were unaffected by increased levels of cAMP. Receptor responses were affected however; the maximum response to N-ethylcarboxamidoadenosine (an A2 receptor agonist) was reduced by 30-40%, there was a small but consistent shift to the right of the dose-response curve for iloprost (a stable analogue of prostacyclin) and [3H]iloprost binding studies revealed a loss of prostacyclin receptors. However, the loss of receptor responsiveness was much smaller than that which occurs following pretreatment with prostacyclin or adenosine A2 receptor agonists (Keen et al. (1989) Biochem. Pharmacol. 38, 3827-3833; Kelly et al. (1990) Br. J. Pharmacol. 99, 309-316) suggesting that cAMP may not play a major role in agonist mediated desensitization.

Epoxyeicosatrienoic acid stimulates ADP-ribosylation of a 52 kDa protein in rat liver cytosol

In rat liver cytosol, rapid ADP-ribosylation of a 52 kDa protein by endogenous ADP-ribosyltransferase(s) was observed. This ADP-ribosylation was stimulated dose-dependently by 14,15-epoxyeicosatrienoic acid (14,15-EET), one of the metabolites of arachidonic acid by NADPH-dependent cytochrome P-450 mono-oxygenase. This stimulatory effect required the presence of GTP or its non-hydrolysable analogues, guanosine 5'-[beta gamma-imido]triphosphate or guanosine 5'-[gamma-thio]triphosphate. Of four regioisomeric EETs, 14,15-EET was the most potent. No stimulatory effect was observed with addition of 14,15-dihydroxyeicosatrienoic acid, a stable metabolite of 14,15-EET. The 52 kDa protein was not ADP-ribosylated by cholera toxin A subunit and pertussis toxin, and was not recognized by anti-Gs alpha and anti-Gi alpha antibodies. However, the 52 kDa protein could be photoaffinity-labelled with 8-azidoguanosine 5'-[alpha-32P]triphosphate. These results suggest that the 52 kDa protein is neither Gs nor Gi, though it may have a GTP-binding site. These results contribute to the understanding of the role of mono-oxygenase metabolites of arachidonic acid in intracellular signal transduction.

Endogenous ADP-ribosylation in brain: initial characterization of substrate proteins

Cholera and pertussis toxin-mediated ADP-ribosylation has been used extensively to study regulation of guanine nucleotide binding proteins (G proteins) in the nervous system, but much less is known about possible endogenous ADP-ribosylation of G proteins in brain. The present study demonstrates endogenous ADP-ribosylation, in the absence of cholera and pertussis toxins, of four predominate proteins in homogenates of rat cerebral cortex. These proteins showed apparent molecular masses of 20, 42, 45, and 50 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 42- and 45-kDa proteins comigrated precisely with the major cholera toxin-labeled bands. Furthermore, the endogenous ADP-ribosylated and cholera toxin-ADP-ribosylated bands yielded identical 32P-labeled peptide fragments by one-dimensional peptide mapping, indicating that they are probably the same proteins, presumably the alpha-subunits of Gs. In contrast, peptide maps of the 50-kDa protein, which migrated close to a 48-kDa cholera toxin-labeled band, demonstrated that this protein is distinct from the toxin-labeled band and from Gs alpha. Levels of endogenous ADP-ribosylation activity showed regional heterogeneity in brain, with a nearly threefold variation observed among the brain regions examined. Chronic administration (7 days) of corticosterone significantly increased overall levels of endogenous ADP-ribosylation, indicating that components of this system may be under hormonal control in vivo. Attempts to identify neurotransmitters or second messenger systems that regulate endogenous ADP-ribosylation activity in brain have so far been unsuccessful with one exception.(ABSTRACT TRUNCATED AT 250 WORDS)

CTP-dependent endogenous ADP-ribosylation of a 38 kDa protein in HL-60 cell membranes

Incubation of membranes of human promyelocytic leukemia HL-60 cells with [32P]NAD led to ADP-ribosylation of several proteins including a 38 kDa protein by endogenous ADP-ribosyltransferases. The ADP-ribosylation of the 38 kDa protein was distinctly different from others on the basis of pH dependency and heat stability at 50 degrees C, suggesting that there are at least two endogenous ADP-ribosyltransferases. It was enhanced by CTP, but not affected by ATP, GTP and UTP, whereas it was inhibited by GTP gamma S. [alpha-32P]CTP bound to the 38 kDa protein immobilized on a nitrocellulose sheet, indicating that the 38 kDa protein which bound CTP is strongly ADP-ribosylated by an endogenous ADP-ribosyltransferase.

Agonist-induced ADP-ribosylation of a cytosolic protein in human platelets

alpha-Thrombin and phorbol 12,13-dibutyrate stimulated the mono(ADP-ribosyl)ation of a 42-kDa cytosolic protein of human platelets. This effect was mediated by protein kinase C activation and was inhibited by protein kinase C inhibitor staurosporine. It also was prevented by prostacyclin, which is known to inhibit the phospholipase C-induced formation of 1,2-diacylglycerol, which is one of the endogenous activators of protein kinase C. On sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the 42-kDa protein that is ADP-ribosylated by alpha-thrombin was clearly distinct from the alpha subunits of membrane-bound inhibitory and stimulatory guanine nucleotide-binding regulatory proteins, respectively Gi alpha and Gs alpha; the 47-kDa protein that is phophorylated by protein kinase C in platelets; and the 39-kDa protein that has been shown to be endogenously ADP-ribosylated by agents that release nitric oxide. This information shows that agonist-induced activation of protein kinase leads to the ADP-ribosylation of a specific protein. This covalent modification might have a functional role in platelet activation.