Pertussis toxin inhibits fMet-Leu-Phe- but not phorbol ester-stimulated changes in rabbit neutrophils: role of G proteins in excitation response coupling

Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca2+ channels activity through a differential mechanism

Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca²⁺ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca²⁺ channels in the absence as in the presence of extracellular Ca²⁺. Here, we investigate the mechanism underlying the modulation of Ca²⁺-independent Ca²⁺ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca²⁺, the Mn²⁺ has been used as a Ca²⁺ surrogate to determine the activity of Ca²⁺-independent Ca²⁺ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca²⁺-independent Ca²⁺ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La³⁺-sensitive Ca²⁺ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca²⁺ channels not sensitive to La³⁺, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca²⁺ channels by phosphorylation/de-phosphorylation-dependent mechanisms.

G protein and diacylglycerol regulate metamorphosis of planktonic molluscan larvae

The regulatory guanine nucleotide binding protein (G protein) activators cholera toxin and the GTP analog 5-guanylyl imidophosphate, the second messenger diacylglycerol, and certain diamino acids all facilitate (amplify) the settlement and metamorphic responses of planktonic larvae of Haliotis rufescens (marine mollusc) to morphogenetic chemical stimuli. In contrast, the G protein-inhibiting analog guanosine 5'-O-[beta-thio]diphosphate inhibits facilitation by L-alpha,beta-diaminopropionic acid but does not block facilitation by diacylglycerol. Diacylglycerol, cholera toxin, and the guanine nucleotide analogs alone neither induce the settlement and metamorphosis of the larvae nor do they inhibit induction of metamorphosis by gamma-aminobutyric acid. These results thus establish the existence of separate regulatory and inductive pathways controlling larval metamorphosis in response to two classes of exogenous chemical signals from the environment. The regulatory pathway, operating independently through a G protein-diacylglycerol cascade apparently controlled by facilitating diamino acids in the water column, can amplify the larval responsiveness to inducers of metamorphosis. This mechanism may have adaptive significance in the recognition and selection of favorable habitats for metamorphosis of the larvae. Similar regulatory pathways, based on exogenous control of a G protein-diacylglycerol cascade, may govern responsiveness to stimuli in other sensory and developmental systems.

Mechanisms Involved in the Enhancement of Allergic Airways Neutrophil Influx by Permanent C-Fiber Degeneration in Rats

This study was undertaken to clarify the mechanisms by which C-fiber degeneration at neonatal stages exacerbates the inflammatory responses of rat airways. Rats were treated with capsaicin at neonatal stages and immunized with ovalbumin (OVA) at adult ages. Challenge of capsaicin-pretreated rats with OVA promoted a higher influx of neutrophils in bronchoalveolar lavage (BAL) fluid compared with the vehicle group. No significant differences were found for the other cell types. The increased adhesion of N-formyl-methionyl-leucyl-phenylalanine (fMLP; 0.1 microM)- and phorbol myristate acetate (PMA; 1 microM)-treated neutrophils to fibronectin-coated wells did not differ among vehicle- and capsaicin-pretreated rats. Additionally, fMLP (10 microM), platelet-activating factor (0.1 microM), and substance P (50 microM) induced a significant neutrophil chemotaxis, but no differences were found among vehicle and capsaicin groups. Increased levels of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-10, and leukotriene B4 in BAL fluid as well as higher expression of cytokine-induced neutrophil chemoattractant (CINC)-3 in lung homogenates were detected in the capsaicin group compared with vehicle group. In the capsaicin group, chronic treatment with compound 48/80 restored the TNF-alpha levels to control values and prevented the neutrophil influx in BAL fluid. The enhanced production of TNF-alpha, superoxide anion, and nitrite by isolated alveolar macrophages in response to lipopolysaccharide (3 microg/ml), PMA (10 nM), and/or zymosan (100 particles/cell) did not differ between vehicle- and capsaicin-pretreated rats. In conclusion, chronic neuropeptide depletion promoted by neonatal capsaicin treatment up-regulates airways mast cells, which upon activation by antigen at adult ages, release large amounts of cytokines such as TNF-alpha and CINC-3 that accounts for the massive airways neutrophil infiltration.

The human formyl peptide receptor as model system for constitutively active G-protein-coupled receptors

According to the two-state model of G-protein-coupled receptor (GPCR) activation, GPCRs isomerize from an inactive (R) state to an active (R*) state. In the R* state, GPCRs activate G-proteins. Agonist-independent R/R* isomerization is referred to as constitutive activity and results in an increase in basal G-protein activity, i.e. GDP/GTP exchange. Agonists stabilize the R* state and further increase, whereas inverse agonists stabilize the R state and decrease, basal G-protein activity. Constitutive activity is observed in numerous wild-type GPCRs and disease-causing GPCR mutants with increased constitutive activity. The human formyl peptide receptor (FPR) exists in several isoforms (FPR-26, FPR-98 and FPR-G6) and activates chemotaxis and cytotoxic cell functions of phagocytes through G(i)-proteins. Studies in HL-60 leukemia cell membranes demonstrated inhibitory effects of Na(+) and pertussis toxin on basal G(i)-protein activity, suggesting that the FPR is constitutively active. However, since HL-60 cells express several constitutively active chemoattractant receptors, analysis of constitutive FPR activity was difficult. Sf9 insect cells do not express chemoattractant receptors and G(i)-proteins and provide a sensitive reconstitution system for FPR/G(i)-protein coupling. Such expression studies showed that FPR-26 is much more constitutively active than FPR-98 and FPR-G6 as assessed by the relative inhibitory effects of Na(+) and of the inverse agonist cyclosporin H on basal G(i)-protein activity. Site-directed mutagenesis studies suggest that the E346A exchange in the C-terminus critically determines dimerization and constitutive activity of FPR. Moreover, N-glycosylation of the N-terminus seems to be important for constitutive FPR activity. Finally, we discuss some future directions of research.

Identification of diacylated ureas as a novel family of fungus-specific leukocyte-activating pathogen-associated molecules

Polymorphonuclear leukocytes represent primary components of the host's innate immune defenses against fungal infection, suggesting involvement of fungal leukocyte attractants. We have found in various fungi, but not in bacteria or host cells, unstable lipid-like leukocyte chemoattractants, which also induced adherence and degranulation in human neutrophils. Purification from bakers' yeast and structural analyses by electrospray mass spectrometry, (1)H NMR spectroscopy, and chemical synthesis revealed these inflammatory mediators as diacylated ureas, a novel class of unstable lipoids. The N,N'-dipalmitoleyl urea appeared to be the most potent innate immune responses inducing compound eliciting half-maximum neutrophil chemotactic activity at 140 nm. The all-trans isomer, N,N'-dipalmitelaidyl urea, was found to be inactive with respect to stimulation of degranulation in neutrophils, which indicates a Delta(9) cis-double bond to be essential for bioactivity of these diacyl ureas. N,N'-Dipalmitoleyl urea elicited Ca(2+) mobilization in neutrophils, which was found to be pertussis toxin-sensitive and sensitive toward a carboxylmethyltransferase inhibitor, indicating that these diacyl ureas activate leukocytes via a putative Galpha(i)-protein-coupled receptor. Their isolation exclusively from fungi suggests that these lipoids are fungus-specific pathogen-associated molecules that may alert the human innate immunity system to the presence of a fungal infection.

High constitutive activity of the human formyl peptide receptor

The formyl peptide receptor (FPR) couples to pertussis toxin (PTX)-sensitive Gi-proteins to activate chemotaxis and exocytosis in neutrophils. PTX reduces not only formyl peptide-stimulated but also agonist-independent ("basal") Gi-protein activity, suggesting that the FPR is constitutively active. We aimed at identifying an inverse FPR agonist, i.e. a compound that suppresses constitutive FPR activity. In Sf9 insect cell membranes, the G-protein heterotrimer Gialpha2beta1gamma2 reconstituted N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-stimulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTPgammaS-sensitive high affinity [3H]FMLP binding. The FPR "antagonist" cyclosporin H (CsH) potently and efficiently reduced basal GTPgammaS binding in Sf9 membranes. Another FPR antagonist, N-t-butoxycarbonyl-L-phenylalanyl-L-leucyl-L-phenylalanyl-L-leucyl-L- phenylalanine did not inhibit basal GTPgammaS binding but blocked the inhibitory effect of CsH on GTPgammaS binding. Na+ reduced basal GTPgammaS binding and eliminated the inhibitory effect of CsH. Similar effects of FMLP, CsH, and Na+ as in Sf9 membranes were observed with FPR expressed in the mammalian cell line HEK293. Our data show that the human FPR possesses high constitutive activity. CsH is an inverse FPR agonist and stabilizes the FPR in an inactive state. Na+ also stabilizes the FPR in an inactive state and, thereby, diminishes inverse agonist efficacy.

The G alpha i homologue gna-1 controls multiple differentiation pathways in Neurospora crassa

Heterotrimeric G proteins are components of principal signaling pathways in eukaryotes. In higher organisms, alpha subunits of G proteins have been divided into four families, Gi, Gs, Gq, and G12. We previously identified a G alpha i homologue gna-1 in the filamentous fungus Neurospora crassa. Now we report that deletion of gna-1 leads to multiple phenotypes during the vegetative and sexual cycles in N. crassa. On solid medium, delta gna-1 strains have a slower rate of hyphal apical extension than wild type, a rate that is more pronounced under hyperosmotic conditions or in the presence of a cellophane overlay. delta gna-1 mutants accumulate less mass than wild-type strains, and their mass accumulation is not affected in the same way by exposure to light. delta gna-1 strains are defective in macroconidiation, possessing aerial hyphae that are shorter, contain abnormal swellings, and differentiate adherent macroconidia. During the sexual cycle, delta gna-1 strains are fertile as males. However, the mutants are female-sterile, producing small, aberrant female reproductive structures. After fertilization, delta gna-1 female structures do not enlarge and develop normally, and no sexual spores are produced. Thus, mutation of gna-1 results in sex-specific loss of fertility.

Quantitative contribution of the acid production to the intracellular acidification in human neutrophils stimulated by N-formyl-methionyl-leucyl-phenylalanine

A chemotactic peptide, N-formyl-methionyl-leucyl-phenylalanine (fMLP), induced an acidification of cytosol by about 0.05 pH units in 30 sec followed by an alkalinization in human neutrophils. The quantitative contribution of acid production to the acidification was studied. The superoxide (O2-) production stimulated by fMLP was not involved in the acidification because the production of acids in neutrophils from patients with chronic granulomatous disease who do not produce O2-, was the same as that in normal neutrophils. The intracellular acidification was completely inhibited by deoxyglucose, suggesting that energy metabolism enhanced upon stimulation by fMLP might be the main source of the acidification. Although enhancement of the lactate formation by fMLP was 0.8 nmol/10(6) cells, which could lower intracellular pH by 0.08 pH units, the lactate production could not explain the initial acidification because the production of lactate started at 1 min after the stimulation while the intracellular acidification began immediately after the stimulation. Mitochondrial respiratory inhibitors such as KCN and rotenone had no effects on the fMLP-induced intracellular acidification. The fMLP-induced production of CO2 in 30 sec through the hexose monophosphate shunt was only 2.6 pmol/10(6) cells, which was calculated to decrease intracellular pH by only 0.0014. Thus, changes of energy metabolism induced by fMLP does not explain the acidification.

Pentoxifylline and CD14 antibody additively inhibit priming of polymorphonuclear leukocytes for enhanced release of superoxide by lipopolysaccharide: possible mechanism of these actions

Lipopolysaccharide (LPS) primes human polymorphonuclear leukocytes (PMN) for enhanced O2- production in response to stimulation by N-formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe). Serum factor is essential for priming at lower concentrations of LPS. Complexes of LPS and LPS-binding protein are recognized by CD14 on PMN. We investigated the effects of a monoclonal antibody against CD14 (MY4) and of pentoxifylline (POF), a membrane fluidizer, alone and in combination, on LPS-LPS-binding protein activation of phospholipase D evidenced by increased phosphatidic acid formation. Phosphatidic acid formation and O2- production were inhibited by MY4 and POF. Our results suggest that the actions of these agents occur at an early step in the excitation-response sequence. In the absence of a second stimulus, LPS plus serum caused an increase in the amount of Gi alpha 2 associated with the membrane via CD14. POF, however, had no effect on Gi alpha 2 in the membrane. POF alone significantly changed the affinity (KD) of the fMet-Leu-Phe receptor of PMN (from 25.2 +/- 4.5 nM to 15.2 +/- 2.4 nM [P < 0.01; n = 4]) at 37 degrees C. The differences between the sites of action of MY4 and POF may lead to cooperation by these agents for inhibition of priming by LPS plus serum for enhanced O2- production. Clinical use of the antibody and POF may diminish tissue damage caused by PMN in clinical endotoxic shock.

Neutrophil recruitment in Clostridium difficile toxin A enteritis in the rabbit

Neutrophil infiltration is a prominent feature of Clostridium difficile-associated enteritis and colitis. The aim of this study was to examine the importance of neutrophil recruitment and neutrophil-mediated tissue damage in C. difficile toxin A-induced enteritis. Competitive binding experiments using purified 3H-toxin A demonstrated the presence of a single class of medium affinity receptors on rabbit neutrophils (Kd 7 x 10(-8) M). Pertussis toxin and the nonhydrolyzable GTP analog GTPgamma S both inhibited 3H-toxin A binding (by 56 and 65%, respectively), indicating that the rabbit neutrophil toxin A receptor is G protein linked. Toxin A elicited a dose-dependent (25-200 micrograms/ml) stimulation of neutrophil migration in vitro, and this functional effect was also pertussis toxin sensitive (69% inhibition). Treatment of neutrophils with R15.7, a blocking monoclonal antibody to the leuocyte adhesion molecule CD18, inhibited toxin A-stimulated neutrophil migration by 85% in vitro. Pretreatment of rabbits with R15.7 also prevented neutrophil infiltration of toxin A-exposed ileal loops in vivo as determined by histologic examination and by ileal tissue myeloperoxidase levels. Furthermore, R15.7 effected a substantial inhibition of fluid secretion (by 65%), mannitol permeability (by 66%), and histologic damage in toxin A-exposed ileal loops. Anti-CD18 (R15.7) had no inhibitory effect on cholera toxin enterotoxicity. These data demonstrate that C. difficile toxin A is a proinflammatory toxin whose enterotoxic effects are substantially dependent upon neutrophil recruitment.

Regulation of Clostridium perfringens alpha-toxin-activated phospholipase C in rabbit erythrocyte membranes

The rapid phosphatidic acid (PA) formation induced by Clostridium perfringens alpha-toxin was stimulated by AlF4- in rabbit erythrocyte membranes. GTP[gamma S] [guanosine 5'-O-(3-thiotriphosphate)] stimulated the rapid 1,2-diacylglycerol formation and inositol 1,4,5-trisphosphate release induced by the toxin. On the other hand, treatment of erythrocyte lysates with phorbol 12-myristate 13-acetate (PMA) resulted in inhibition of toxin-induced PA production, and long-term PMA or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) treatment of the lysates led to stimulation of PA formation. Furthermore, treatment of erythrocytes with the toxin caused an increase of protein kinase C activity in membrane fractions. The results suggest that toxin-induced PA formation is mediated by endogenous phospholipase C regulated through GTP-binding protein and protein kinase C in rabbit erythrocytes.

Linkage of azurophil granule secretion in neutrophils to chloride ion transport and endosomal transcytosis

Neutrophils contain at least two types of secretory granules. The present work links the secretion of the (lysosomal type) azurophil granules, but not that of specific granules, to endosomal transport mechanisms. (a) Selective stimulation of azurophil granule secretion by the Na-ionophore Monensin, or nonselective stimulation by FMLP after cytochalasin B pretreatment elicited marked pinocytic activity in parallel with azurophil granule release, whereas FMLP alone, selective for specific granules, elicited little fluid pinocytosis. (b) Pinosomes thus formed fused with azurophil granules, suggesting that exocytosis of azurophil granules might occur via endosomal organelles. This hypothesis was tested by determining the effect on the endosomal pathway(s) of two treatments that selectively prevent the release of azurophil granule contents without interfering with specific granule secretion, namely replacement of Cl- with gluconate- or the addition of zinc. Replacement of Cl- was found to impair the pinocytosis process itself, whereas ZnSO4 appeared to prevent the fusion between endosomes and azurophil granules. These data support the concept that the (lysosomal type) azurophil granules, but not the specific granules, are secreted through the endosomal pathway.

Differential cholera-toxin- and pertussis-toxin-catalysed ADP-ribosylation of G-proteins coupled to formyl-peptide and leukotriene B4 receptors

N-Formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe) and leukotriene B4 (LTB4) induce disparate second-messenger generation and functional responses in neutrophils and HL-60 granulocytes. Receptors for these chemoattractants couple to a common pool of G-proteins which are substrates for both pertussis-toxin- and cholera-toxin-catalysed ADP-ribosylation. The hypothesis that formyl-peptide and LTB4 receptors induce different receptor-specific conformations of activated G-proteins was tested. The ability of pertussis toxin and cholera toxin to ADP-ribosylate G(i) proteins coupled to formyl-peptide or LTB4 receptors in membranes isolated from HL-60 granulocytes was used to assess the conformational state of the alpha subunits. Cholera-toxin-catalysed ADP-ribosylation of alpha 40 (40 kDa alpha subunit) was inhibited by guanosine 5'-[beta gamma-imido]triphosphate and GDP in a concentration-dependent manner. Addition of fMet-Leu-Phe, but not LTB4, re-established cholera-toxin labelling of alpha 40 in the presence of either guanine nucleotide. In the absence of guanine nucleotides, fMet-Leu-Phe and C5a enhanced cholera-toxin-catalysed labelling of alpha 40, whereas LTB4 and platelet-activating factor had no effect. Preincubation with fMet-Leu-Phe, but not LTB4, inhibited pertussis-toxin labelling of alpha 40 in the presence of guanosine 5'-[gamma-thio]triphosphate and in the absence of guanine nucleotides. Preincubation with fMet-Leu-Phe or LTB4 enhanced pertussis-toxin labelling of alpha 40 in the presence of GDP. These data suggest that activated G(i) proteins coupled to formyl-peptide and LTB4 receptors exist in different conformations determined by the receptor with which they interact.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Host mediators in gingival crevicular fluid: implications for the pathogenesis of periodontal disease

During the past few years, a considerable number of studies have examined different aspects of the host response in gingival crevicular fluid (GCF), including the relationship of specific markers to the active phases of periodontal disease. Various indicators of the acute inflammatory response (the lysosomal enzymes beta-glucuronidase and collagenase, the cytoplasmic enzyme aspartate aminotransferase, and the arachidonic acid metabolite PGE2) have been shown to be associated with clinical attachment loss in chronic adult periodontitis in man and experimental periodontitis in animal models. In contrast, the relationship of indicators of the humoral immune response in GCF to active periodontal disease is equivocal. Furthermore, a number of indicators of the cellular immune response have been identified recently in GCF (i.e., Interleukin-1 alpha, IL-1 beta, tumor necrosis factor-alpha), but their relationship to active phases of periodontal disease have not been studied. The polymorphonuclear leukocyte (PMN) is the cellular hallmark of acute inflammation. Evidence from the GCF studies suggests that hyperreactivity of these cells plays a critical role in the active phases of some forms of periodontal disease. Metabolic activation of PMN can be associated with a number of potentially destructive reactions. The major effector mechanism for tissue destruction that can be specifically identified with the PMN is the synergistic effect of the release of PMN proteases and the generation of reactive oxygen metabolites by these cells. Priming of the PMN, where the PMN response is enhanced by agents that do not initiate the response, may be an important mechanism for PMN activation in the crevicular environment; for example, cytokines such as IL-1 beta and TNF-alpha, and lipopolysaccharides released from subgingival Gram-negative bacteria, can serve this function. The hypothesis proposed here argues that in addition to the severe forms of periodontal disease that have been associated with qualitative or quantitative PMN defects, tissue destruction in the periodontum can be observed with hyperreactivity of these cells. These differing conclusions do not create a dilemma, but may represent opposite ends of a balance that is no longer in equilibrium.

Receptor class desensitization of leukocyte chemoattractant receptors

To better define their regulation, formylpeptide and C5a chemoattractant receptor cDNAs were transiently expressed with high efficiency (approximately 35-54%) in human kidney cells. As in neutrophils, both receptors were active in elevating intracellular calcium (ED50 approximately 0.5-1 nM). Agonist-specific desensitization for calcium elevation was observed for both chemoattractant receptors at doses of approximately 1 nM. Heterologous desensitization of formylpeptide, C5a, and alpha 1-adrenergic receptors required high doses of phorbol ester (100 nM phorbol 12-myristate 13-acetate). To further study the phenomenon of desensitization, formylpeptide and C5a receptor cDNAs were cotransfected resulting in approximately 80% of receptor-positive cells expressing both receptors. These cells also possessed endogenous alpha 1-adrenergic receptors. Interestingly, chemoattractant receptors were cross-desensitized by pretreatment with low doses of either C5a or formylmethionylleucylphenylalanine (10 nM) but not by the alpha 1-adrenergic agonist norepinephrine (up to 10 microM). Neither chemoattractant desensitized alpha 1-adrenergic receptors. This phenomenon was reproduced in human neutrophils. These data suggest a previously uncharacterized mechanism of receptor regulation, which is intermediate between homologous and heterologous desensitization. Class desensitization of chemoattractant receptors is less selective than homologous desensitization but is far more efficient and specific than heterologous desensitization. Receptor class desensitization may affect functional classes of receptors via modification of either the receptor or the shared guanine nucleotide-binding regulatory protein.

Effects of protein I of Neisseria gonorrhoeae on neutrophil activation: generation of diacylglycerol from phosphatidylcholine via a specific phospholipase C is associated with exocytosis

Upon engagement of chemoattractant receptors, neutrophils generate inositol trisphosphate and diacylglycerol (DG) by means of a phosphatidylinositol-specific phospholipase C (PI-PLC) which is regulated by a GTP-binding protein(s). We have previously reported (Reibman, J., H. M. Korchak, L. B. Vosshall, K. A. Haines, A. M. Rich, and G. Weissmann. 1988. J. Biol. Chem. 263:6322-6328) a biphasic rise in DG after exposure of neutrophils to the chemoattractant FMLP: a rapid (less than or equal to 15 s) phase ("triggering") and a slow (greater than or equal to 30 s) phase ("activation"). These derive from distinct intracellular lipid pools. To study the source of rapid and slow DG, we have used a unique probe, protein I, a porin that is the major outer membrane protein of Neisseria gonorrhoeae. Treatment of neutrophils with protein I inhibits exocytosis and homotypic cell adhesion provoked by FMLP without inhibiting assembly of the NADPH oxidase responsible for O2-. generation. DG turnover in PMN labeled with [3H]arachidonate and [14C]glycerol was profoundly altered by protein I. Whereas the rapid peak of DG was only modestly diminished (FMLP vs. FMLP plus protein I = DG labeled with [3H]arachidonic acid (3H-a.a.-DG): 142 +/- 14% SEM vs. 125 +/- 22%; DG labeled with the glycerol backbone with [14C]glycerol (D-14C-G): 125 +/- 10% SEM vs. 107 +/- 8.5% SEM), the slow rise in both 3H-a.a.-DG and D-14C-G was essentially abolished. Moreover, treatment of neutrophils with 4-4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), which, like protein I, inhibits exocytosis without affecting O2-. generation also inhibited slow DG. However, protein phosphorylation and dephosphorylation (47phox, 66phox) were unaffected in the absence of slow DG. To determine the source of the slow DG, we have analyzed radiolabeled phospholipid (PL) turnover after FMLP +/- protein I (P.I.). Treatment of PMN with FMLP (0.1 microM) resulted in breakdown of phosphatidylcholine (PC), beginning at 30 s, and reaching a nadir at 60 s (3H-PC = 59 +/- 10.2% SEM of resting, 14C-PC = 57 +/- 6.4%). Protein I (0.25 microM) significantly inhibited PC turnover after FMLP ([3H]PC = 95 +/- 5.6% and [14C]PC = 86 +/- 8.4% of resting at 60 s), but failed to alter the metabolism of 3H- or 14C-phosphatidylinositol after FMLP (91 +/- 19.6 and 88 +/- 16.5% vs. 92 +/- 9.2 and 91 +/- 16% at 60 s).(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of a histone H4 protein kinase in Triton X-100 lysates of rabbit peritoneal neutrophils pretreated with chemotactic factors: lack of requirements of calcium mobilization and protein kinase C activation

The characteristics of the activation of a histone H4 kinase activity in Triton X-100 lysates of rabbit peritoneal neutrophils pretreated with fMet-Leu-Phe were studied: The activation of the kinase was a) inhibited by the antagonist of formylpeptide, t-Boc-(Phe-Leu)2(-)-Phe, b) completely inhibited by pertussis toxin pretreatment, c) not affected by the pretreatment of neutrophils with an activator of protein kinase C, phorbol-12-myristate-13-acetate, or an inhibitor of protein kinase C, 1-(5-isoquinoline-sulfonyl)-2-methyl-piperazine, and d) not inhibited in the cells preloaded with the intracellular calcium chelators, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetra acetic acid acetoxymethyl-ester (BAPTA/AM). These results suggest that the stimulus-induced activation of H4 kinase requires functional receptor and GTP-binding protein but neither calcium mobilization nor protein kinase C activation.

Muscarinic cholinergic stimulation of inositol phosphate production in cultured embryonic chick atrial cells. Evidence for a role of two guanine-nucleotide-binding proteins

These studies demonstrate a novel mechanism for the coupling of the muscarinic receptor to phospholipase C activity in embryonic chick atrial cells. In monolayer cultures of atrial cells from hearts of embryonic chicks at 14 days in ovo, carbamylcholine stimulated the sequential appearance of InsP3, InsP2 and InsP1 with an EC50 (concn. causing 50% of maximal stimulation) of 30 microM. In the presence of 15 mM-Li, a 5 min exposure to carbamylcholine (0.1 mM) increased InsP3 levels to a maximum of 47 +/- 12% over basal, InsP2 to 108 +/- 13% over basal and InsP1 to 42 +/- 5% over basal. This effect was blocked by 5 microM-atropine. Incubation of these cells with pertussis toxin (15 h; 0.5 ng/ml) inhibited carbamylcholine-stimulated InsP3, InsP2 and InsP1 formation by 42 +/- 7%, 30 +/- 3% and 48 +/- 7% respectively. The IC50 (concn. causing 50% inhibition) for pertussis toxin inhibition of all three inositol phosphates was 0.01 ng/ml, with a half-time of 6 h at 0.5 ng/ml. This partial sensitivity to pertussis toxin was not due to incomplete ADP-ribosylation of the guanine-nucleotide-binding protein (G-protein), since autoradiography of polyacrylamide gels of cell homogenates incubated with [32P]NAD+ in the presence of pertussis toxin demonstrated that incubation of cells with 0.5 ng of pertussis toxin/ml for 15 h resulted in complete ADP-ribosylation of pertussis toxin substrates by endogenous NAD+. In cells permeabilized with saponin (10 micrograms/ml), 0.1 mM-GTP[S] (guanosine 5'-[gamma-thio]triphosphate) stimulated InsP1 by 102 +/- 15% (mean +/- S.E.M., n = 4), InsP2 by 421 +/- 67% and InsP3 by 124 +/- 33% above basal. Incubation of cells for 15 h with 0.5 ng of pertussis toxin/ml decreased GTP[S]-stimulated InsP1 production in saponin-treated cells by 30 +/- 10% (n = 3), InsP2 production by 45 +/- 7% (n = 4) and InsP3 production by 49 +/- 6% (n = 4). These data demonstrate that in embryonic chick atrial cells at least two independent G-proteins, a pertussis toxin-sensitive G-protein and a pertussis toxin-insensitive G-protein, play a role in coupling muscarinic agonist binding to phospholipase C activation and to inositol phosphate production.

Involvement of a phospholipase D in the mechanism of action of granulocyte-macrophage colony-stimulating factor (GM-CSF): priming of human neutrophils in vitro with GM-CSF is associated with accumulation of phosphatidic acid and diradylglycerol

The generation of diradylglycerol (DRG) and phosphatidic acid (PdtOH) was investigated in neutrophils primed with granulocyte-macrophage colony-stimulating factor (GM-CSF). Mass accumulation of DRG and PdtOH was measured using reversed-phase high performance liquid chromatography and thin layer chromatography, respectively. GM-CSF had no direct effect on the levels of PdtOH and DRG, but it increased PdtOH generation and the late phase of DRG accumulation in human neutrophils stimulated with FMLP. The elevation of the mass of PdtOH peaked approximately 100 s and clearly preceded that of DRG, which peaked at 150 s. The diacylglycerol kinase inhibitor R59022 enhanced the sustained increase in DRG but did not produce a parallel inhibition in PdtOH production. GM-CSF was without effect on the level of inositol 1,4,5-triphosphate [Ins(1,4,5)P3] and did not affect the liberation of Ins(1,4,5)P3 induced by FMLP. These findings exclude the involvement of the PtdIns(4,5)P2-specific phospholipase C/diacylglycerol pathway in the sustained phase of DRG accumulation. The early (30-s) appearance of PdtOH clearly suggests that GM-CSF enhanced FMLP receptor-linked phospholipase D (PLD) generation of PdtOH. PLD was assessed more directly by formation of labeled phosphatidylethanol (PEt) through PLD capacity of catalyzing a trans-phosphatidylation in presence of ethanol. The formation of PEt associated with a concomitant decrease in PdtOH directly demonstrated that the mechanism by which GM-CSF enhances PdtOH production is activation of a PLD active on phosphatidylcholine. This study provides evidence that the mechanism of action of GM-CSF involves upregulation of PLD activity leading to enhanced generation of PdtOH and DRG in FMLP-stimulated neutrophils. These findings may provide the basis for several of the priming effects of GM-CSF.

Hepoxilin A3 induces changes in cytosolic calcium, intracellular pH and membrane potential in human neutrophils

The effects of hepoxilin A3 (HxA3), a 12-lipoxygenase metabolite of arachidonic acid, on cytosolic calcium ([Ca2+]i), intracellular pH (pHi), transmembrane potential and right-angle light scattering in human neutrophils were investigated. A rapid, transient elevation of [Ca2+]i was observed with HxA3 which was dependent on the concentration used. The effect of HxA3 on [Ca2+]i was blocked by pertussis toxin, suggesting involvement of receptors coupled to GTP-binding proteins. Experiments in Ca2(+)-free medium and using intracellular Ca2+ chelators indicated that HxA3 mobilized Ca2+ from intracellular stores. At similar concentrations, HxA3 altered pHi, producing an initial acidification followed by an alkalinization. The initial acidification was decreased in cells loaded with a Ca2+ chelator. In the presence of N-ethyl-N-(1-methylethyl)amino amiloride, an inhibitor of the Na+/H+ antiport, HxA3 induced a greater acidification but failed to elicit the recovery phase, suggesting that the latter is due to activation of the antiport. HxA3 also depolarized the membrane potential, although this effect was small. A decrease in right-angle light scattering, qualitatively similar to that observed with chemotactic peptides, was seen with HxA3, indicating that the 12-lipoxygenase metabolite can induce shape changes in neutrophils. At the concentrations used for the above effects, HxA3 was unable to generate a respiratory burst. These findings suggest that hepoxilins, which are formed by stimulated neutrophils, may have a role as messengers in neutrophil activation.

Inhibition of neutrophil oxidative burst by elastase-generated IgG fragments

IgG1 is cleaved in vitro by granulocyte elastase into Fc, Fab and Fabc fragments. The cleaved products have been isolated by a series of chromatographic procedures and characterized with regard to molecular mass and isoelectric point. The Fc fragment has been previously shown to express at its N-terminal site a neoantigen which is specific for elastase (Kolb, G., Eckle, I., Heidtmann, H.-H., Neurath, F. & Havemann, K. (1988) Scand. J. Rheumatol. S75, 179-189). The production of superoxide radical anions in prestimulated neutrophils is inhibited dose-dependently by the elastase-generated Fc and Fabc fragments. Native IgG1 and Fab fragments show no inhibitory effect, nor do papain-generated Fc fragments. The degree of inhibition depends on the stimulus applied: half-maximal inhibition is obtained by 6 microM Fc after stimulation with 4 beta-phorbol and 2.4 microM after stimulation with fMet-Leu-Phe; neutrophils stimulated with serum-activated zymosan are not inhibited by IgG fragments. The effect of Fc is purely cellular; no inhibition of O2 generation can be produced by applying Fc to the xanthine oxidase/xanthine system. The fragments have no effect on the activation or activity of crude NADPH oxidase, which is the O2-forming enzyme system of neutrophils. Possible mechanisms are discussed by which Fc acts on stimulated neutrophils.

Dual effects of pertussis toxin on murine neutrophils in vivo. I. Pertussis toxin inhibits extravasation potential of mature neutrophils while simultaneously stimulating granulopoiesis

Pertussis toxin (Ptx) has been employed as an adjuvant by many investigators to augment various types of cell-mediated and humoral immune responses. Recent work from our laboratory indicates that the exacerbation of delayed-type hypersensitivity (DTH) and contact hypersensitivity (CH) responses observed in Ptx-treated mice may be mediated by an absolute increase in the number of circulating neutrophils capable of migrating into tissue sites of antigen challenge. The purpose of the present study was to analyze the effects of Ptx on neutrophils and neutrophil function in vivo. Evidence is presented here suggesting that Ptx has both direct and indirect effects on neutrophils following its in vivo administration to normal mice. Mature neutrophils that are directly exposed to the actions of Ptx in vivo exhibit a marked reduction in their ability to extravasate into tissue sites of inflammation. These findings are consistent with those that have been reported following the exposure of isolated neutrophils to the effects of Ptx in vitro (i.e., that Ptx has an inhibitory effect on many of the functional capabilities of isolated neutrophils). Moreover, we have also determined that Ptx can affect the kinetics of neutrophil production indirectly through its ability to stimulate granulopoiesis. Ptx-exposed mice develop a protracted peripheral blood neutrophilia following toxin administration. Although the mechanism(s) involved in stimulating increased neutrophil production is presently unclear, both dexamethasone and indomethacin (cyclooxygenase pathway inhibitors) are able to function synergistically with Ptx to produce a markedly enhanced neutrophilia in exposed mice. We propose that the capacity of Ptx to augment CH and DTH responses and act as a potent adjuvant may relate, in part, to its ability to alter the rate of neutrophil production in vivo.

Kinetics of cell-free activation of neutrophil NADPH oxidase. Effects of neomycin and guanine nucleotides

The effects of neomycin, fluoride and the non-hydrolysable guanine nucleotide analogue GTP gamma S on the kinetics of cell-free activation of NADPH oxidase in membranes of resting human neutrophils were investigated. Arachidonate-mediated activation of the oxidase followed a first-order reaction course (kobs. = 0.39 min-1 at 26 degrees C). In the presence of NaF during the activation process, activity was enhanced while the activation rate was slightly reduced (kobs. = 0.25 min-1 at 26 degrees C). Neomycin blocked activation (half-maximal effect at 25 microM) without affecting rates of superoxide release by preactivated enzyme in vitro or in vivo. In spite of reduced specific activity neither the first-order rate constant of the activation nor the Km of the oxidase were altered by neomycin. Oxidase activated in the presence of GTP gamma S exhibited increased specific activity and unchanged Km; the course of the reaction deviated from first-order kinetics. Kinetic evidence is presented for two separate activation reactions: a GTP gamma S-independent, basal, first-order process and a GTP gamma S-dependent sigmoid activation process. The results are compatible with the existence in neutrophil membranes of two separate pools of dormant oxidase. An alternative scheme of the formation of two active forms of NADPH oxidase is also presented.

Influence of bacterial toxins and forskolin upon vasopressin-induced inositol phosphate accumulation in WRK 1 cells

The accumulation of inositol phosphates in WRK 1 cells, stimulated with a range of vasopressin concentrations, was diminished by prior exposure to cholera toxin or forskolin, whilst that observed in the presence of maximal concentrations of the hormone was enhanced in pertussis-toxin-treated cells. In the presence of [32P]NAD+, both cholera toxin and pertussis toxin provoked the labelling of peptides with approximate Mrs of 45,000 and 41,000 respectively in the membranes of WRK 1 cells. Exposure to cholera toxin or forskolin for 15-18 h enhanced cyclic AMP accumulation in these cells. The concentrations of these agents which provoked half-maximal cyclic AMP accumulation were similar to those required to diminish receptor-mediated inositol phosphate accumulation by 50%. In contrast, half-maximal ADP-ribosylation of the 45,000Mr peptide needed 100-fold greater concentrations of the toxin than were effective in provoking half-maximal inhibition of inositol phosphate accumulation. Cholera toxin or forskolin also reduced the maximal specific binding, to intact WRK 1 cells, of both [3H][Arg8]vasopressin and the V1a antagonist [3H][beta-mercapto-beta,beta-cyclopentamethylenepropionic acid,O-methyl-Tyr2, Arg8]vasopressin. The kinetics for the loss of this binding capacity following cholera-toxin treatment were very similar to those describing the diminution of vasopressin-stimulated inositol phosphate accumulation in the same cells.

Evidence that activation of a common G-protein by receptors for leukotriene B4 and N-formylmethionyl-leucyl-phenylalanine in HL-60 cells occurs by different mechanisms

Differentiated HL-60 cells were found to respond to the chemoattractants leukotriene B4 (LTB4) and N-formylmethionyl-leucyl-phenylalanine (FMLP), in a manner similar to neutrophils. Membranes of myeloid differentiated HL-60 cells were used (a) to examine the ability of LTB4 receptors to interact with a guanine-nucleotide-binding protein (G-protein), and (b) to compare this G-protein with that which is coupled to the FMLP receptor. LTB4 stimulated a dose-dependent increase in GTP hydrolysis and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) binding, demonstrating that LTB4 receptors on HL-60 cells are coupled to a G-protein. Both pertussis toxin and cholera toxin inhibited stimulation of GTPase activity and GTP[S] binding by either LTB4 or FMLP, indicating that both receptors are coupled to a G-protein containing a 40 kDa alpha-subunit. That the two receptors share a common G-protein was shown by FMLP enhancement of cholera-toxin-induced inhibition of GTPase activity stimulated by either FMLP or LTB4. However, LTB4 did not enhance cholera-toxin-induced inhibition of GTPase activity, suggesting that the receptors interacted differently with this G-protein. This difference was confirmed by showing that FMLP, but not LTB4, stimulated receptor-specific [32P]ADP-ribosylation of the 40 kDa alpha-subunit. Concentrations of LTB4 and FMLP which produced maximal responses produced enhanced stimulation in both assays. This additive effect was not abolished by inactivation of up to 80% of G-protein activity by N-ethylmaleimide or cholera toxin. We conclude that LTB4 and FMLP receptors in HL-60 cells are coupled to a common G-protein. The receptor--G-protein interaction is different for the two receptors, and G-proteins not coupled to both receptors may account for the additive response.

Characterization of the membrane-associated GTPase activity: effects of chemotactic factors and toxins

Membranes prepared from rabbit neutrophils exhibit GTPase activity which can be stimulated by the chemotactic factor fMet-Leu-Phe. The maximum contribution of the ATPase activities to the basal and the fMet-Leu-Phe-stimulated GTPase activities are less than 20% and 9%, respectively. The basal GTPase activity has a Vmax = 34.2 +/- 1.3 (pmol/mg protein, min) and a Km = 0.39 +/- 0.03 microM; and the fMet-Leu-Phe-stimulated has a Vmax = 52.3 +/- 2.5 (pmol/mg protein, min), and a Km = 0.29 +/- 0.02 microM. The GTPase activity can be stimulated by fMet-Leu-Phe and leukotriene B4. Unlike these two chemotactic factors, concanavalin A does not stimulate this GTPase activity. In addition, the rise in intracellular concentration of free calcium produced by concanavalin A is not inhibited by pertussis toxin treatment. Both the basal and stimulated GTPase activities are affected by pertussis toxin, cholera toxin and N-ethylmaleimide.

Protein kinases in neutrophils: a review

In chemotactic factor-stimulated neutrophils, rapid increases of intracellular levels of cyclic AMP, calcium, and diacylglycerol have been observed and may be linked to protein kinase activation. The study of the physiological role and regulation of protein kinases in the neutrophil and the identification of their substrates has provided valuable information on the molecular mechanism of neutrophil activation. The focus of this review is on those aspects of protein kinases that are relevant to neutrophil activation and on the substrate proteins for these protein kinases. The possible role of protein phosphorylation in neutrophil function is also discussed.

Purification of two distinct types of phosphoinositide-specific phospholipase C from rat liver. Enzymological and structural studies

Two kinds of phosphoinositide-specific phospholipase C (PLC) were purified from rat liver by acid precipitation and several steps of column chromatography. About 50% of the activity could be precipitated when the pH of the liver homogenate was lowered to pH 4.7. The redissolved precipitate yielded two peaks, PLC I and PLC II, in an Affi-gel Blue column, and each was further purified to homogeneity by three sequential h.p.l.c. steps, which were different for the two enzymes. The purified PLC I and PLC II had estimated Mr values of 140,000 and 71,000 respectively on SDS/polyacrylamide-gel electrophoresis. Both enzymes hydrolysed phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) in a Ca2+- and pH-dependent manner. PLC I was most active at 10 microM- and 0.1 mM-Ca2+ for hydrolysis of PI and PIP2 respectively, whereas PLC II showed the highest activity at 5 mM- and 10 microM-Ca2+ for that of PI and PIP2 respectively. The optimal pH of the two enzymes also differed with substrates or Ca2+ concentration, in the range pH 5.0-6.0. Hydrolysis of phosphoinositides by these enzymes was completely inhibited by Hg2+ and was affected by other bivalent cations. From data obtained by peptide mapping and partial amino acid sequencing, it was clarified that PLC I and PLC II had distinct structures. Moreover, partial amino acid sequences of three proteolytic fragments of PLC I completely coincided with those of PLC-148 [Stahl, Ferenz, Kelleher, Kriz & Knopf (1988) Nature (London) 332, 269-272].

The dynamics of exocytosis in human neutrophils

We have investigated the dynamics of exocytosis in single human neutrophils. The increase of membrane area associated with granule fusion was followed by time-resolved patch-clamp capacitance measurements. Intracellular application of 20 microM guanosine-5'-O(3-thiotriphosphate) (GTP gamma S) in the presence of 2.5 mM ATP stimulated exocytosis and led to an increase of membrane capacitance from 3.0 to integral of 8.4 pF corresponding to a 540 micron 2 increase of membrane area. This capacitance change is very close to the value expected from morphological data if all primary and secondary granules fuse with the plasma membrane. High resolution measurements revealed stepwise capacitance changes corresponding to the fusion of individual granules. GTP gamma S-stimulated exocytosis did not require pretreatment with cytochalasin B and the amplitude was independent of the intracellular-free calcium concentration between 10 nM and integral of 2.5 microM. In the absence of GTP gamma S elevation of intracellular-free calcium concentration to the micromolar range led to the fusion of only a limited number of granules. Degranulation stimulated with GTP gamma S started after a lag phase of 2-7 min and was usually complete within 5-20 min. The time course was affected by the intracellular ATP and calcium concentration. Exocytosis was markedly accelerated by pretreatment with cytochalasin B. Our results demonstrate that the final steps leading to primary and secondary granule fusion are controlled by a guanine nucleotide-binding protein and do not require an elevation of intracellular calcium. Calcium and other factors are, however, involved in the regulation having pronounced effects on the dynamics of exocytosis.

Signal transduction in cells following binding of chemoattractants to membrane receptors

Binding of chemoattractants to specific cell surface receptors on human polymorphonuclear leukocytes (PMNs) initiates a variety of biologic responses, including directed migration (chemotaxis), release of superoxide anions, and lysosomal enzyme secretion. Chemoattractant receptors belong to a large class of receptors which utilize the hydrolysis of polyphosphoinositides to initiate Ca2+ mobilization and cellular activation. Receptor occupancy leads to phospholipase C-mediated hydrolysis of polyphosphoinositol 4,5-bisphosphate (PIP2) yielding inositol 1,4,5-trisphosphate (IP3) and 1,2 sn-diacylglycerol (DAG). These products synergize to initiate cell activation via calcium mobilization (IP3) and protein kinase C activation (DAG). Pertussis toxin, which ADP-ribosylates and inactivates some GTP binding proteins (G proteins), abolishes all chemoattractant-induced responses, including Ca2+ mobilization, IP3 and DAG production, enzyme secretion, superoxide production and chemotaxis. Direct evidence for chemoattractant receptor: G protein coupling was obtained using PMN membrane preparations which contain a Ca2+-sensitive phospholipase C. Hydrolysis of polyphosphoinositides at resting intracellular Ca2+ levels (100 nm) was only observed when the membranes were stimulated with the chemoattractant N-formyl-methyl-leucyl-phenylalanine (fMet-Leu-Phe) in the presence of GTP. Myeloid cells contain two distinct pertussis toxin substrates of similar molecular weight (40 and 41 kD). The 41 kD substrate resembles Gi, whereas a 40 kD substrate is physically associated with a partially purified fMet-Leu-Phe receptor preparation and may therefore represent a novel G protein involved in chemoattractant-stimulated responses. Metabolism of 1,4,5-IP3 to inositol proceeds via two distinct pathways in PMNs: (1) degradation to 1,4-IP2 and 4-IP1 or (2) conversion to 1,3,4,5-IP4, 1,3,4-IP3, 3,4-IP2 and 3-IP1. Initial formation (0-30 s) of 1,4,5-IP3 and DAG occurs at ambient intracellular Ca2+ levels, whereas formation of 1,3,4-IP3 and a second sustained phase of DAG production (30 s-10 min) require elevated cytosolic Ca2+ influx. The later peak of DAG, which is not derived from phosphoinositides, appears to be required for stimulation of respiratory burst activity. Products formed during activation can feed back to attenuate chemoattractant receptor-mediated stimulation of phospholipase C by uncoupling receptor-G protein-phospholipase C interaction.

Inhibition of formation of cyclic AMP and cyclic GMP by vasopressin in smooth-muscle cells is insensitive to pertussis toxin

Pretreatment of A-10 cells with pertussis toxin had no effect on [arginine]vasopressin-mediated inhibition of cyclic nucleotide accumulation. Pretreatment of the cells with the same concentration of pertussis toxin produced 90-95% inhibition of [32P]ADP ribosylation in membranes, suggesting that these cells possess pertussis-toxin substrate and that the toxin enters the cells to reach its site of action. The functional integrity of the pertussis-toxin substrate in these cells is confirmed by the observation that in these cell membranes increasing concentrations of GTP inhibited basal, forskolin- and NaF-stimulated adenylate cyclase activities, and this inhibition was abolished when the cells were pretreated with pertussis toxin. In addition, thrombin-mediated inhibition of isoprenaline-stimulated cyclic AMP accumulation was also inhibited by pertussis-toxin pretreatment of the cells. These data suggest that, unlike thrombin, [arginine]vasopressin-induced inhibitory effects on cyclic nucleotide accumulation in smooth-muscle cells are not mediated by pertussis-toxin substrate.

Evidence for a GTP-binding protein involved in interferon-gamma transduction signal

These studies were designed to investigate the characteristics of the intracellular messengers induced by interferons (IFN-alpha/beta and IFN-gamma) after receptor binding. Pretreatment of target cells with V. cholerae toxin, which is known to activate a membrane GTP-binding stimulatory protein (Gs), potentiated the action of IFN-gamma, but not of IFN-alpha/beta. By contrast, B. pertussis toxin, which is known to activate the GTP-binding inhibitory protein (Gi), had no effects on the action of both IFN-alpha/beta and IFN-gamma. Further support to the involvement of G proteins in IFN-gamma transduction signal came from the finding that a non-hydrolizable GTP analog, GTP-gamma-S, enhanced in the presence of phorbol esters (PMA) the antiviral and antiproliferative activity of IFN-gamma, but not of IFN-alpha/beta. On the other hand, forskolin or PGE1, known to increase the intracellular cAMP levels by different metabolic pathways, when added together with IFN-gamma, significantly potentiated its antiviral and antiproliferative activity. Pretreatment of the cultures with the above drugs completely prevented IFN-gamma activity. No effects were observed when forskolin or PGE1 were used with IFN-alpha/beta. Finally, the modulation of IFN-gamma activity by the above drugs was not a consequence of changes in the expression of the specific surface receptors, since [125I]IFN-gamma binding by pretreated target cells was comparable to that of untreated cultures. Altogether these results demonstrate that the IFN-gamma, but not the IFN-alpha/beta, transduction signal is mediated after receptor binding by a G protein with functional characteristics similar to those of the known Gs proteins. Activation of the adenylate cyclase system could be one of the subsequent steps involved in IFN-gamma action.

Control of glomerulosa cell function by angiotensin II: transduction by G-proteins and inositol polyphosphates

1. The receptor-activated mechanisms that mediate the steroidogenic actions of angiotensin II (AII) have been characterized in rat and bovine adrenal glomerulosa cells. In rat adrenal cells, the AII receptor is coupled to a guanine nucleotide inhibitory protein which reduces adenylate cyclase activity and cyclic AMP production. However, receptor-mediated stimulation of aldosterone production by AII is exerted through a separate pertussis-insensitive nucleotide regulatory protein that subserves coupling of activated receptors to phospholipase C. 2. In AII-stimulated glomerulosa cells, hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP2) by phospholipase C yields diacylglycerol and inositol 1,4,5-trisphosphate (Ins-P3), which act as second messengers by activating calcium-calmodulin and calcium-phospholipid dependent protein kinase pathways. Ins-1,4,5-P3 is a potent stimulus of intracellular calcium mobilization, and is promptly inactivated by two major routes of metabolism. Direct degradation of Ins-1,4,5-P3 by a 5-phosphatase gives inositol 1,4-bisphosphate which in turn is metabolized to inositol-4-monophosphate. The latter product can be derived only from higher inositol phosphates, and thus serves as a specific marker of polyphosphoinositide breakdown in agonist-stimulated cells. In contrast, inositol-1-phosphate is largely derived from phosphatidylinositol hydrolysis, which is not increased during the initial phase of AII action. 3. Ins-1,4,5-P3 formed in AII-stimulated glomerulosa cells is also phosphorylated by a calcium-calmodulin dependent 3-kinase to form inositol 1,3,4,5-tetrakisphosphate (Ins-P4), which is rapidly dephosphorylated to the biologically inactive Ins-1,4,5-P3 isomer, Ins-1,3,4-trisphosphate. The latter metabolite, like Ins-1,4,5-P3, is both degraded to lower phosphates (Ins-3,4,P2 and Ins-1,3-P2) and phosphorylated to form a new tetrakisphosphate isomer (Ins-1,3,4,6-P4). Ins-1,4,5-P3 formed during AII action is bound with high affinity to specific intracellular receptors through which InsP3 causes calcium mobilization during the initiation of cellular responses to AII and other calcium-dependent ligands.