Chemoattractant-stimulated GTPase activity is decreased on membranes from polymorphonuclear leukocytes incubated in chemoattractant

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

Chemotaxis Receptors and Signaling

Chemotaxis is an important cellular response common in biology. In many chemotaxing cells the signal that regulates movement is initiated by G protein-coupled receptors on the cell surface that bind specific chemoattractants. These receptors share important structural similarities with other G protein-coupled receptors, including rhodopsin, which currently serves as the best starting point for modeling their structures. However, the chemotaxis receptors also share a number of relatively unique structural features that are less common in other GPCRs. The chemoattractant ligands of chemotaxis receptors exhibit a broad variety of sizes and chemical properties, ranging from small molecules and peptides to protein ligands. As a result, different chemotaxis receptors have evolved specialized mechanisms for the early steps of ligand binding and receptor activation. The mechanism of transmembrane signaling is currently under intensive study and several alternate mechanisms proposing different conformational rearrangements of the transmembrane helices have been proposed. Some chemotaxis receptors are proposed to form dimers, and in certain cases dimer formation is proposed to play a role in transmembrane signaling. In principle the structural and dynamical changes that occur during transmembrane signaling could be specialized for different receptors, or could be broadly conserved. Extensive mutagenesis studies have been carried out, and have begun to identify critical residues involved in ligand binding, receptor activation, and transmembrane signaling.

Requirements for C5a receptor-mediated IL-4 and IL-13 production and leukotriene C4 generation in human basophils

Anaphylatoxin derived from the fifth complement component (C5a) in the presence of IL-3 induces continuous leukotriene C4 generation and IL-4 and IL-13 expression in human basophils for a period of 16-18 h. This indicates that the G protein-coupled C5a receptor (C5aR) can induce long-lasting cellular responses. Using anti-N-terminal C5aR Abs, C-terminal C5a hexapeptide analogs, and pertussis toxin, we demonstrate that the putative activation site of the C5aR is both necessary and sufficient for these late cellular responses. Furthermore, continuous pertussis toxin-sensitive G protein-coupled receptor activation and receptor-ligand interaction is ongoing and required during the entire period of product release. However, the late basophil responses have a more stringent requirement for optimal receptor activation. Leukotriene C4 generation appears to be influenced mostly by the way the receptor is activated, because the most active hexapeptide is a superagonist for this response. By contrast, C5adesarg, lacking the C-terminal arginine, induces minimal lipid mediator formation but is fully active to induce IL-4 production and is even a superagonist for IL-13 release. Nevertheless, IL-4/IL-13 synthesis in response to C5adesarg could be blocked by both C-terminal antagonistic peptide as well as anti-N-terminal C5aR Abs, indicating only minor differences of ligand-receptor interactions between C5a and C5adesarg. Taken together, our data demonstrate that long-lasting and continuous signaling occurs through a limited activation domain of the C5aR, which can differentially promote separate basophil functions.

Deficient homologous desensitization of formyl peptide receptors stably expressed in undifferentiated HL-60 cells

The ability of formyl peptide receptors (FPRs) stably expressed in undifferentiated HL-60 cells to undergo ligand-induced desensitization was compared with their ability in normal and vector-transfected HL-60 cells following granulocyte differentiation with DMSO. fMet-Leu-Phe failed to induce uncoupling of FPRs from G-proteins in FPR-transfected cells, whereas uncoupling was induced in differentiated HL-60 cells and differentiated vector-transfected HL-60 cells, as determined by ligand-stimulated guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) binding and GTPgammaS inhibition of fMet-Leu-Phe binding to isolated membranes. Immunoprecipitation of Galpha(i2) from solubilized, azidoanalide (AA-gammaGTP) photolabeled membranes showed that receptors in desensitized FPR-transfected HL-60 cells remained coupled to Galpha(i2), whereas desensitized receptors in differentiated HL-60 cell membranes were uncoupled from Galpha(i2). As determined by immunoblotting, Galpha(i2) expression was similar in undifferentiated and differentiated HL-60 cells and FPR-transfected cells. Ligand-stimulated receptor internalization and desensitization of calcium redistribution were similar in all three groups of cells. Immunoblotting also indicated that G-protein-coupled receptor kinases (GRKs) 2 and 4 were present in undifferentiated FPR-transfected HL-60 cells at 50% of the level seen in differentiated HL-60 cells. However, differentiation did not increase GRK2 or GRK4 expression, indicating that differences in GRK expression do not explain deficient desensitization. The data indicated that undifferentiated HL-60 cells are unable to induce homologous desensitization of FPRs.

Integrating conflicting chemotactic signals. The role of memory in leukocyte navigation

Leukocytes navigate through complex chemoattractant arrays, and in so doing, they must migrate from one chemoattractant source to another. By evaluating directional persistence and chemotaxis during neutrophil migration under agarose, we show that cells migrating away from a local chemoattractant, against a gradient, display true chemotaxis to distant agonists, often behaving as if the local gradient were without effect. We describe two interrelated properties of migrating cells that allow this to occur. First, migrating leukocytes can integrate competing chemoattractant signals, responding as if to the vector sum of the orienting signals present. Second, migrating cells display memory of their recent environment: cells' perception of the relative strength of orienting signals is influenced by their history, so that cells prioritize newly arising or newly encountered attractants. We propose that this cellular memory, by promoting sequential chemotaxis to one attractant after another, is in fact responsible for the integration of competitive orienting signals over time, and allows combinations of chemoattractants to guide leukocytes in a step-by-step fashion to their destinations within tissues.

Neutrophils stimulated with a variety of chemoattractants exhibit rapid activation of p21-activated kinases (Paks): separate signals are required for activation and inactivation of paks

Activation of the p21-activated protein kinases (Paks) was compared in neutrophils stimulated with a wide variety of agonists that bind to receptors coupled to heterotrimeric G proteins. Neutrophils stimulated with sulfatide, a ligand for the L-selectin receptor, or the chemoattractant fMet-Leu-Phe (fMLP), platelet-activating factor, leukotriene B4, interleukin-8, or the chemokine RANTES exhibited a rapid and transient activation of the 63- and 69-kDa Paks. These kinases exhibited maximal activation with each of these agonists within 15 s followed by significant inactivation at 3 min. In contrast, neutrophils treated with the chemoattractant and anaphylatoxin C5a exhibited a prolonged activation (>15 min) of these Paks even though the receptor for this ligand may activate the same overall population of complex G proteins as the fMLP receptor. Addition of fMLP to neutrophils already stimulated with C5a resulted in the inactivation of the 63- and 69-kDa Paks. Optimal activation of Paks could be observed at concentrations of these agonists that elicited only shape changes and chemotaxis in neutrophils. While all of the agonists listed above triggered quantitatively similar activation of the 63- and 69-kDa Paks, fMLP was far superior to the other stimuli in triggering activation of the c-Jun N-terminal kinase (JNK) and the p38 mitogen-activated protein kinase (MAPK). These data indicate that separate signals are required for activation and inactivation of Paks and that, in contrast to other cell types, activated Pak does not trigger activation of JNK or p38-MAPK in neutrophils. These results are consistent with the recent hypothesis that G-protein-coupled receptors may initiate signals independent of those transmitted by the alpha and betagamma subunits of complex G proteins.

Multistep navigation and the combinatorial control of leukocyte chemotaxis

Cells migrating within tissues may encounter multiple chemoattractant signals in complex spatial and temporal patterns. To understand leukocyte navigation in such settings, we have explored the migratory behavior of neutrophils in model scenarios where they are presented with two chemoattractant sources in various configurations. We show that, over a wide range of conditions, neutrophils can migrate "down" a local chemoattractant gradient in response to a distant gradient of a different chemoattractant. Furthermore, cells can chemotax effectively to a secondary distant agonist after migrating up a primary gradient into a saturating, nonorienting concentration of an initial attractant. Together, these observations suggest the potential for cells' step-by-step navigation from one gradient to another in complex chemoattractant fields. The importance of such sequential navigation is confirmed here in a model system in which neutrophil homing to a defined domain (a) requires serial responses to agonists presented in a defined spatial array, and (b) is a function of both the agonist combination and the sequence in which gradients are encountered. We propose a multistep model of chemoattractant-directed migration, which requires that leukocytes display multiple chemoattractant receptors for successful homing and provides for combinatorial determination of microenvironmental localization.

Phosphorylation of the N-formyl peptide receptor is required for receptor internalization but not chemotaxis

The human N-formyl peptide receptor (FPR) is a member of the family of leukocyte, G protein-coupled, chemoattractant receptors. To determine the role(s) of receptor phosphorylation in FPR processing and formylmethionylleucylphenylalanine (fMLF)-mediated chemotaxis, we utilized U937 cells expressing the recombinant wild type receptor and a mutant form of the FPR. This mutant, which lacks all of the serine and threonine residues in the C terminus of the receptor, DeltaST, has recently been shown to produce a receptor capable of fMLF binding and G protein activation but was demonstrated not to undergo fMLF-dependent phosphorylation or desensitization of the calcium mobilization response upon repeated exposure to agonist (Prossnitz, E. R. (1997) J. Biol. Chem. 272, 15213-15219). In this report, we examined the role of receptor phosphorylation in FPR internalization and leukocyte chemotaxis. Whereas the wild type receptor was rapidly internalized upon stimulation, the phosphorylation-deficient mutant was not, remaining entirely on the cell surface. In addition, contrary to the hypothesis that receptor processing and recycling are required for chemotaxis, we found no defect in the ability of the mutant FPR to migrate up a concentration gradient of fMLF. These results indicate that phosphorylation of the FPR is a necessary step in receptor internalization but that receptor phosphorylation, desensitization, and internalization are not required for chemotaxis.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

Desensitization of N-formylpeptide receptor-mediated activation is dependent upon receptor phosphorylation

The human N-formylpeptide receptor (FPR) represents one of the most thoroughly studied leukocyte chemoattractant receptors. Despite this, little is known about the molecular mechanisms involved in the activation and desensitization of this receptor. To assess the role of phosphorylation in receptor function, U937 promonocytic cells were stably transfected to express the recombinant human FPR. Three mutant forms of the FPR lacking specific serine and threonine residues in the receptor C terminus were studied with respect to activation and desensitization. Replacement of all 11 serine and threonine residues within the C terminus by alanine and glycine residues (DeltaST) resulted in a receptor capable of ligand binding and G protein activation similar to the wild-type receptor. However, whereas the wild-type FPR was phosphorylated on both serine and threonine residues upon exposure to agonist and displayed a significantly reduced ability to stimulate G protein-mediated GTP hydrolysis upon subsequent exposure to agonist, DeltaST demonstrated a complete lack of phosphorylation and displayed little alteration in its ability to stimulate G protein-mediated GTP hydrolysis upon a subsequent exposure to agonist. In addition to desensitization of G protein-mediated GTP hydrolysis, calcium mobilization was assayed to test whether desensitization occurred at a site distal to G protein activation. However, as observed with G protein activation, DeltaST underwent no desensitization of the calcium mobilization response upon a second exposure to agonist. To define more precisely the role of specific serine and threonine residues, two additional mutants were analyzed. Replacement either of Ser328, Thr329, Thr331, and Ser332 (mutant A) or of Thr334, Thr336, Ser338, and Thr339 (mutant B) resulted in functional receptors that exhibited approximately 50% the level of phosphorylation following stimulation. Whereas mutant A, like DeltaST, could not be significantly desensitized by exposure to agonist, mutant B exhibited partial desensitization. These results indicate that phosphorylation of the FPR is a necessary and sufficient step in cellular desensitization, that multiple phosphorylation sites are involved, and that redundant desensitization does not occur downstream of G protein activation in the signaling cascade.

Fixation traps formyl peptide receptors in high and low affinity forms that can be regulated by GTP[S] in the absence of ligand

The formyl peptide receptor on human neutrophils recognizes bacterial, N-formylated peptides and initiates a cascade of intracellular signals via a pertussis toxin sensitive Gi protein. We used fluorescence techniques to investigate the interactions of ligand (L), receptor (R), and G proteins (G), the ternary complex, in both live and fixed human neutrophils. By lightly fixing permeabilized neutrophils with a procedure that retained ligand binding, we were able to "capture' R and G in different configurations in the absence of ligand. Fixed receptors were trapped in a high affinity form (attributed to LRG) that could not be rapidly converted to low affinity by the addition of GTP[S]. Adding saturating nucleotide prior to fixation trapped receptors in a low affinity form (attributed to LR). The low affinity receptors retained the sensitivity of the native receptors to the presence of NA+. The distribution between high and low affinity receptors was modulated by GTP[S] in a dose dependent manner. The ability to redistribute low and high affinity receptor forms prior to fixation was unique to GTP[S], as compared to other non-activating nucleotides, suggesting that GTP[S] can regulate the distribution between R and RG. We suggest that precoupled receptors that give rise to high affinity ligand binding are likely to exist in native membranes in human neutrophils.

Cross-desensitization of chemoattractant receptors occurs at multiple levels. Evidence for a role for inhibition of phospholipase C activity

To define the molecular mechanisms of cross-regulation among chemoattractant receptors, we stably coexpressed, in a rat basophilic leukemia (RBL-2H3) cell line, epitope-tagged receptors for the chemoattractants formylmethionylleucylphenylalanine (fMLP), a peptide of the fifth component of the complement system (C5a), and interleukin-8 (IL-8). All the expressed receptors underwent homologous phosphorylation and desensitization upon agonist stimulation. When co-expressed, epitope-tagged C5a receptor (ET-C5aR) and epitope-tagged IL-8 receptor (ET-IL-8RA) were cross-phosphorylated by activation of the other. Activation of epitope-tagged fMLP receptor (ET-FR) also cross-phosphorylated ET-C5aR and ET-IL-8RA, but ET-FR was totally resistant to cross-phosphorylation. Similarly, C5a and IL-8 stimulation of [35S]guanosine 5'-3-O-(thio) triphosphate (GTP gamma S) binding and Ca2+ mobilization were cross-desensitized by each other and by fMLP. Stimulation of [35S]GTP gamma S binding by fMLP was also not cross-desensitized by C5a or IL-8, however, Ca2+ mobilization was, suggesting a site of inhibition distal to G protein activation. Consistent with this desensitization of Ca2+ mobilization, inositol 1,4,5-trisphosphate release in RBL-2H3 cells expressing both ET-C5aR and ET-FR revealed that fMLP and C5a cross-desensitized each other's ability to stimulate phosphoinositide hydrolysis. Taken together, these results indicate that receptor cross-phosphorylation correlates directly with desensitization at the level of G protein activation. The ET-FR was resistant to this process. Of note, cross-desensitization of ET-FR at the level of phosphoinositide hydrolysis and Ca2+ mobilization was demonstrated in the absence of receptor phosphorylation. This suggests a new form of chemoattractant cross-regulation at a site distal to receptor/G protein coupling, involving the activity of phospholipase C.

Ethanol affects leukotriene generation and leukotriene-induced functional responses in human polymorphonuclear granulocytes

Since ethanol has been shown to inhibit the inflammatory response, we evaluated whether ethanol affected generation of leukotrienes in polymorphonuclear granulocytes (PMN) in vitro. Using the calcium ionophore A23187 as stimulus, the leukotriene B4 (LTB4) and leukotriene C4 (LTC4) generation were dose-dependently impaired by ethanol. No significant difference in the levels of the omega-oxidized metabolites was observed. However, the total LTB4 production (LTB4 plus omega-oxidized metabolites) was significantly decreased in the samples treated with ethanol. Furthermore, ethanol also modulated LTB4-induced functional responses. PMN aggregation, oxidative metabolism and elastase release were all inhibited in the presence of 1% ethanol (to 74 +/- 15%, 50 +/- 4% and 57 +/- 3% of controls, respectively). However, ethanol had no effect on intracellular calcium mobilization or on the change of the PMN membrane potential induced by either LTB4 or A23187. Thus, a possible mechanism for the reduced functional PMN responses in the presence of ethanol might be impaired generation of leukotrienes, but it is conceivable that ethanol impairs also other steps of the stimulus response coupling since the LTB4-induced functional responses were inhibited.

Identification of the major phosphorylation sites in human C5a anaphylatoxin receptor in vivo

Interaction of human C5a anaphylatoxin with cell surface receptors mediates cell activation and receptor desensitization. Treatment of differentiated HL60 cells or transiently transfected COS-7 cells with C5a or phorbol 12-myristate 12-acetate (PMA) results in rapid hyperphosphorylation of the C5aR. In an attempt to gain more insight into the function of phosphorylation in the desensitization of C5aR, we have initiated experiments to identify phosphoacceptor sites at the amino acid level after stimulation of cells with either C5a or PMA. In this report we show that C5aR is phosphorylated exclusively on serine residues in both differentiated HL60 and transfected COS-7 cells irrespective of the stimulus used. Peptide mapping after cyanogen bromide cleavage of phosphorylated C5aR indicates that despite the presence of a protein kinase C consensus motif the third cytoplasmic loop is not phosphorylated when cells are challenged with either C5a or PMA. Thus, whether the cells are stimulated with C5a or PMA, the phosphorylation sites appear to be restricted to serine residues in the carboxyl tail. Phosphoamino acid analysis of a series of mutants in which an individual serine residue was replaced by a threonine residue indicates that the C5aR undergoes C5a-dependent phosphorylation to the maximal stoichiometry of 6 mol of PO4/mol of receptor at Ser314, Ser317, Ser327, Ser332, Ser334, and Ser338. Simultaneous substitution of serine residues by alanine at positions 332, 334, and 338 affected neither the binding of C5a nor the cell surface expression of the mutant, but resulted in a dramatic reduction (more than 80%) of both C5a- and PMA-mediated phosphorylation as compared to the wild type receptor. This result suggests that phosphorylation on the segment extending from Ser332 to Ser338 is required for the subsequent phosphorylation of the carboxyl-terminal tail of C5aR.

Farnesyl thiotriazole, a potent neutrophil agonist and structurally novel activator of protein kinase C

Farnesylcysteine derivatives can initiate or inhibit superoxide (O2-) release in neutrophils. The mechanism by which one of these derivatives, farnesyl thiotriazole (FTT), initiates O2- release in neutrophils is the subject of this paper. Treatment of guinea pig neutrophils with FTT results in the rapid release of O2- by a route shown to be independent of the chemotactic peptide N-formyl-Met-Leu-Phe (fMLP) receptor. The signal transduction pathway utilized by the chemoattractant fMLP is generally accepted as the paradigm for receptor-mediated stimulation of O2- production. Antagonists of fMLP had no effect on FTT-induced O2- release, and pretreatment of neutrophils with fMLP had no effect on the ability of FTT to trigger further O2- generation. In fact, FTT behaves like a typical protein kinase C (PKC) activator. It promotes phosphorylation of the 47-kDa subunit of the NADH oxidase complex (p47-phox) in neutrophils, and this phosphorylation is specifically blocked by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), an antagonist of PKC. FTT is also shown to activate PKC in vitro in a specific and saturable fashion. FTT is approximately equipotent with (S)-diolein, a physiologically relevant activator of this kinase. FTT represents a new, and quite novel, structure for a PKC activator. PKC activators include diglycerides and the structurally diverse tumor promoters.

Temporal segregation in signalling: a novel mechanism in human neutrophils

The ability of neutrophils to carry out chemotaxis in response to low chemoattractant concentrations, but arrest their motility when exposed to higher concentrations of the same substance, has fascinated investigators for years. By analyzing the temporal characteristics of the morphological responses, corresponding to chemotaxis and cell arrest, we have recently discovered that the choice between them is made by transduction of the continuous binding process into either single or multiple stimuli within defined time intervals, initiating chemotaxis or cell arrest, respectively. Both experimental and theoretical lines of evidence are presented to support the validity of this unique mechanism.

Phosphorylation of the N-formyl peptide receptor carboxyl terminus by the G protein-coupled receptor kinase, GRK2

Attenuation of receptor-mediated signal amplification in response to external stimuli, an essential step in the balance of cellular activation, may be mediated by receptor phosphorylation. We have recently shown that the carboxyl-terminal cytoplasmic domain of the N-formyl peptide receptor (FPR) interacts with G proteins and demonstrate here that this same region of the FPR is specifically phosphorylated by a neutrophil cytosolic kinase with properties similar to the G protein-coupled receptor kinase, GRK2. Both kinase activities show a lack of sensitivity toward protein kinase A, protein kinase C, and tyrosine kinase inhibitors but demonstrate almost identical sensitivity toward the kinase inhibitor heparin. Kinetic studies demonstrated that GRK2 has a Km for the carboxyl-terminal domain of the FPR of approximately 1.5 microM and that denaturation of the substrate results in an almost complete loss of phosphorylation. Comparative studies reveal that GRK3 has approximately 50% of the activity of GRK2 toward the FPR carboxyl terminus, whereas GRK5 and GRK6 have no detectable activity. Site-directed mutagenesis of numerous regions of the FPR carboxyl terminus demonstrated that, whereas Glu326/Asp327 and Asp333 are critical for phosphorylation, the carboxyl-terminal 10 amino acids are not required. Simultaneous substitution of Thr334, Thr336, Ser338, and Thr339 resulted in an approximately 50% reduction in phosphorylation, whereas simultaneous substitution of the upstream Ser328, Thr329, Thr331, and Ser332 or merely the Ser328 and Thr329 residues resulted in an approximately 80% reduction in phosphorylation. The introduction of negatively charged glutamate residues for Ser328 and Thr329 or Thr331 and Ser332 resulted in marked stimulation of phosphorylation. These results suggest a hierarchical mechanism in which phosphorylation of amino-terminal serine and threonine residues is required for the subsequent phosphorylation of carboxyl-terminal residues. These results provide the first direct evidence that an intracellular domain of a chemoattractant receptor is a high affinity substrate for GRK2 and further suggest a role for GRK2 or a closely related kinase in the attenuation of receptor-mediated activation of inflammatory cells.

Physical coupling of N-formyl peptide chemoattractant receptors to G protein is unaffected by desensitization

Desensitization of N-formyl peptide chemoattractant receptors (FPR) in human neutrophils results in association of these receptors to the membrane skeleton. This is thought to be the critical event in the lateral segregation of receptors and guanyl nucleotide-binding proteins (G proteins) within the plane of the plasma membrane resulting in an interruption of the signaling cascade. In this study we probed the interaction of FPR with G protein in human neutrophils that were desensitized to various degrees. Human neutrophils were desensitized using the photoreactive agonist N-formyl-met-leu-phe-lys-N epsilon-[125I]2(p-azidosalicylamino)ethyl-1,3'- dithiopropionate (fMLFK-[125I]ASD). The interaction of FPR with protein was studied via a reconstitution assay and subsequent analysis of FPR-G protein complexes in sucrose density gradients. FPR-G protein complexes were reconstituted with solubilized FPR from partially and fully desensitized neutrophils with increasing concentrations of Gi purified from bovine brain. The respective EC50 values for reconstitution were similar to that determined for FPR from unstimulated neutrophils (Bommakanti RK et al., J Biol Chem 267: 7576-7581, 1992). We conclude, therefore, that the affinity of the interaction of FPR with G protein is not affected by desensitization, consistent with the model of lateral segregation of FPR and G protein as a mechanism of desensitization.

Desensitization by protein kinase C activation differentially uncouples formyl peptide receptors from effector enzymes in HL-60 granulocytes

The hypothesis that protein kinase C (PKC) participates in agonist-mediated desensitization of formyl peptide receptors in HL-60 granulocytes was tested. fMet-Leu-Phe and leukotriene B4(LTB4) produced homologous desensitization of agonist-stimulated intracellular calcium transients. Pre-treatment with the PKC activator, phorbol myristate acetate (PMA; 10 nM), abolished both fMet-Leu-Phe and LTB4-stimulated calcium transients. Membranes prepared from control HL-60 granulocytes (NM) or cells treated with 10 nM PMA (PMA-M) demonstrated increased formyl peptide receptor and G protein density, as determined by radioligand binding and pertussis toxin- and cholera toxin-catalysed ADP ribosylation. fMet-Leu-Phe stimulation of guanosine 5'-[gamma-thio]-triphosphate (GTP gamma S) binding and GTP hydrolysis and GDP inhibition of fMet-Leu-Phe binding were not different between NM and PMA-M. Pre-treatment with 10 nM PMA did not inhibit subsequent fMet-Leu-Phe-stimulated superoxide generation or phospholipase D activation. We conclude that PKC desensitizes fMet-Leu-Phe-stimulated phospholipase C, but not phospholipase D, responses and that PKC activation does not mediate agonist-induced desensitization of formyl peptide receptors.

Adenosine inhibits fMLP-stimulated adherence and superoxide anion generation by human neutrophils at an early step in signal transduction

The ability of physiological concentrations of adenosine to inhibit formylmethionylleucylphenylalanine (fMLP)-stimulated superoxide anion (O2-) generation, adherence and degranulation is well established in human neutrophils. However, the mechanism of inhibition remains to be determined. To better understand where adenosine blocks the fMLP signal transduction pathway, we examined the ability of adenosine to inhibit neutrophil adherence stimulated by phorbol myristate acetate (PMA), NaF, and A23187; these agents activate intermediate steps in fMLP signal transduction. Adenosine (0.1-100 microM) did not inhibit adherence mediated by these receptor-independent agonists or NaF- and A23187-mediated O2- production. Additionally, NaF and A23187 completely abrogated adenosine inhibition of fMLP-stimulated neutrophil adherence. We also found that pertussis toxin (5 and 10 microM) completely inhibited fMLP-induced neutrophil adherence and O2- generation, indicating that both processes are G protein mediated. Furthermore, fMLP-stimulated GTPase activity in neutrophil membrane preparations was significantly inhibited by adenosine (1 and 10 microM) or 5'-N-ethylcarboxamidoadenosine (1 microM) (NECA). These data indicate that adenosine inhibits a G-protein-dependent pathway of fMLP stimulation by uncoupling G proteins from the fMLP receptor. This may be a general mechanism of adenosine inhibition of cell-surface receptor-mediated signals as both fMLP- and C5a-stimulated neutrophil adherence were inhibited at similar concentrations.

Activation of a G protein in mouse sperm by the zona pellucida, an egg-associated extracellular matrix

Mammalian sperm possess a guanine nucleotide-binding regulatory protein (G protein), with properties similar to Gi, that appears to be involved in the signal transduction pathway required for zona pellucida (ZP)-mediated acrosomal exocytosis. Mouse sperm treated with pertussis toxin (PT), a toxin that functionally inactivates Gi proteins, bind to the ZP of mouse eggs but are inhibited from undergoing acrosomal exocytosis. We have measured high-affinity GTPase activity and GTP gamma [35S] binding in mouse sperm homogenates incubated in the absence and presence of ZP glycoproteins isolated from either ovulated eggs or from ovarian homogenates to determine whether this extracellular matrix can activate the sperm-associated Gi protein. An increase in GTP hydrolysis (approximately 50% over basal activity) and GTP gamma [35S] binding (approximately 25-60% over basal activity) is observed when sperm homogenates are incubated in the presence of solubilized ZP glycoproteins, and the increase in GTPase activity is dependent on the concentration of ZP added to the homogenates. Accompanying this increase is a reduction in the ability of PT to catalyze in vitro [32P]ADP-ribosylation of a Mr = 41,000 sperm Gi protein, suggesting that the increase in GTPase activity and GTP gamma [35S] binding is associated with the activation of a PT-sensitive sperm G protein(s). The ability of the ZP to stimulate high-affinity GTPase activity in these homogenates appears to be dependent on the capacitation state of the sperm from which the homogenates are prepared. These data suggest that a component(s) of the ZP may function in a manner similar to that of other ligands by binding to a sperm surface-associated receptor and subsequently activating a G protein coupled to an intracellular signal transduction cascade(s) required for induction of acrosomal exocytosis.

Neuropeptides and inflammation. A somatostatin analog as a selective antagonist of neutrophil activation by substance P

OBJECTIVE

Substance P and somatostatin are neuropeptides found in peripheral sensory nerves. In vitro, these have opposing effects on inflammatory cells. We compared the effects of these peptides on the activation of neutrophils.

METHODS

Neutrophils were isolated from healthy volunteers, and chemotaxis, superoxide anion generation, aggregation, and changes in cytosolic calcium and GTPase activity were measured in the presence of substance P, somatostatin, and the chemoattractant FMLP.

RESULTS

Substance P was an effective chemoattractant, 20% as potent as FMLP at equimolar concentrations. Substance P also stimulated GTPase activity in neutrophil plasma membranes. Somatostatin did not activate neutrophils; however, it effectively inhibited neutrophil chemotaxis and GTPase activity provoked by substance P, but not by FMLP.

CONCLUSIONS

These studies demonstrate that substance P can effectively stimulate chemotaxis, possibly via effects on a GTP-binding protein distinct from that triggered by FMLP, and that somatostatin is a selective antagonist of substance P. The biochemical specificities of these peptides on cells may modulate neurogenic inflammation at the local level.

Functional reconstitution of a receptor-activated signal transduction pathway in Xenopus laevis oocytes using the cloned human C5a receptor

We have used the polymerase chain reaction to isolate and clone the cDNA encoding the human C5a receptor, and have injected the cDNA-derived receptor cRNA into Xenopus laevis oocytes for functional characterization of the receptor protein. Receptor activity was determined either electrophysiologically by measuring the agonist-dependent opening of [Ca2+]i-dependent Cl- channels, or by analysing the agonist-dependent efflux of 45Ca2+ from the oocytes. Using both methodologies, injection of pure C5a receptor cRNA failed to confer C5a sensitivity on the oocytes. In contrast, marked responses to C5a were observed when the receptor cRNA was supplemented with poly(A)+ RNA isolated from undifferentiated HL-60 cells, which is devoid of C5a receptor mRNA. Binding studies using radioiodinated C5a revealed that the C5a receptor polypeptide was in fact synthesized and targeted to the oocyte plasma membrane in oocytes injected with receptor cRNA alone, and that the level of receptor expression was not influenced by coinjection of poly(A)+ RNA from undifferentiated HL-60 cells. These results strongly suggest that the human C5a receptor requires a specific cofactor(s) lacking in Xenopus oocytes but present in undifferentiated HL-60 cells, to generate intracellular signals in oocytes. Identification and characterization of this factor will provide important information about the molecular mechanisms by which G-protein-coupled receptors activate phospholipase C.

The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

Characterization of high affinity GTPase activity correlated to beta-adrenergic receptor stimulation of adenylyl cyclase in rat parotid membranes

beta-Adrenergic receptor stimulation of adenylyl cyclase involves the activation of a GTP-binding regulatory protein (G-protein, termed here Gs). Inactivation of this G-protein is associated with the hydrolysis of bound GTP by an intrinsic high affinity GTPase activity. In the present study, we have characterized the GTPase activity in a Gs-enriched rat parotid gland membrane fraction. Two GTPase activities were resolved; a high affinity GTPase activity displaying Michaelis-Menten kinetics with increasing concentrations of GTP, and a low affinity GTPase activity which increased linearly with GTP concentrations up to 10 mM. The beta-adrenergic agonist isoproterenol (10 microM) increased the Vmax of the high affinity GTPase component approx. 50% from 90 to 140 pmol/mg protein per min, but did not change its Km value (approximately 450 nM). Isoproterenol also stimulated adenylyl cyclase activity in parotid membranes both in the absence or presence of GTP. In the presence of a non-hydrolyzable GTP analogue, guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), isoproterenol increased cAMP formation to the same extent as that observed with AlF-4. Cholera toxin treatment of parotid membranes led to the ADP-ribosylation of two proteins (approximately 45 and 51 kDa). Cholera toxin also specifically decreased the high affinity GTPase activity in membranes and increased cAMP formation induced by GTP in the absence or the presence of isoproterenol. These data demonstrate that the high affinity GTPase characterized here is the 'turn-off' step for the adenylyl cyclase activation seen following beta-adrenergic stimulation of rat parotid glands.