Reconstitution of a physical complex between the N-formyl chemotactic peptide receptor and G protein. Inhibition by pertussis toxin-catalyzed ADP ribosylation

Identification of FAM3D as a novel endogenous chemotaxis agonist for the FPRs (formyl peptide receptors)

The family with sequence similarity 3 (FAM3) gene family is a cytokine-like gene family with four members FAM3A, FAM3B, FAM3C, and FAM3D. In this study, we found that FAM3D strongly chemoattracted human peripheral blood neutrophils and monocytes. To identify FAM3D receptor, we used chemotaxis, receptor internalization, calcium flux and radioligand-binding assays in FAM3D-stimulated HEK293 cells that transiently expressed FPR1 or FPR2 to show that FAM3D was a high affinity ligand of formyl peptide receptors (FPR1 and FPR2), both of which were highly expressed on the surface of neutrophils and monocytes/macrophages. After being injected into the mouse peritoneal cavity, FAM3D chemoattracted CD11b+Ly6G+ neutrophils in a short time. In response to FAM3D stimulation, p-ERK and p-p38 were up-regulated in the mouse neutrophils, which could be inhibited by an inhibitor of FPR1 or FPR2. FAM3D was reported to be constitutively expressed in the gastrointestinal tract. We found that FAM3D expression increased significantly in dextran sulfate sodium-induced colitis. Taken together, we propose that FAM3D plays a role in gastrointestinal homeostasis and inflammation through its receptors FPR1 and FPR2.

Identification of C-terminal phosphorylation sites of N-formyl peptide receptor-1 (FPR1) in human blood neutrophils

Accumulation, activation, and control of neutrophils at inflammation sites is partly driven by N-formyl peptide chemoattractant receptors (FPRs). Occupancy of these G-protein-coupled receptors by formyl peptides has been shown to induce regulatory phosphorylation of cytoplasmic serine/threonine amino acid residues in heterologously expressed recombinant receptors, but the biochemistry of these modifications in primary human neutrophils remains relatively unstudied. FPR1 and FPR2 were partially immunopurified using antibodies that recognize both receptors (NFPRa) or unphosphorylated FPR1 (NFPRb) in dodecylmaltoside extracts of unstimulated and N-formyl-Met-Leu-Phe (fMLF) + cytochalasin B-stimulated neutrophils or their membrane fractions. After deglycosylation and separation by SDS-PAGE, excised Coomassie Blue-staining bands (∼34,000 Mr) were tryptically digested, and FPR1, phospho-FPR1, and FPR2 content was confirmed by peptide mass spectrometry. C-terminal FPR1 peptides (Leu(312)-Arg(322) and Arg(323)-Lys(350)) and extracellular FPR1 peptide (Ile(191)-Arg(201)) as well as three similarly placed FPR2 peptides were identified in unstimulated and fMLF + cytochalasin B-stimulated samples. LC/MS/MS identified seven isoforms of Ala(323)-Lys(350) only in the fMLF + cytochalasin B-stimulated sample. These were individually phosphorylated at Thr(325), Ser(328), Thr(329), Thr(331), Ser(332), Thr(334), and Thr(339). No phospho-FPR2 peptides were detected. Cytochalasin B treatment of neutrophils decreased the sensitivity of fMLF-dependent NFPRb recognition 2-fold, from EC50 = 33 ± 8 to 74 ± 21 nM. Our results suggest that 1) partial immunopurification, deglycosylation, and SDS-PAGE separation of FPRs is sufficient to identify C-terminal FPR1 Ser/Thr phosphorylations by LC/MS/MS; 2) kinases/phosphatases activated in fMLF/cytochalasin B-stimulated neutrophils produce multiple C-terminal tail FPR1 Ser/Thr phosphorylations but have little effect on corresponding FPR2 sites; and 3) the extent of FPR1 phosphorylation can be monitored with C-terminal tail FPR1-phosphospecific antibodies.

International Union of Basic and Clinical Pharmacology. LXXIII. Nomenclature for the formyl peptide receptor (FPR) family

Formyl peptide receptors (FPRs) are a small group of seven-transmembrane domain, G protein-coupled receptors that are expressed mainly by mammalian phagocytic leukocytes and are known to be important in host defense and inflammation. The three human FPRs (FPR1, FPR2/ALX, and FPR3) share significant sequence homology and are encoded by clustered genes. Collectively, these receptors bind an extraordinarily numerous and structurally diverse group of agonistic ligands, including N-formyl and nonformyl peptides of different composition, that chemoattract and activate phagocytes. N-formyl peptides, which are encoded in nature only by bacterial and mitochondrial genes and result from obligatory initiation of bacterial and mitochondrial protein synthesis with N-formylmethionine, is the only ligand class common to all three human receptors. Surprisingly, the endogenous anti-inflammatory peptide annexin 1 and its N-terminal fragments also bind human FPR1 and FPR2/ALX, and the anti-inflammatory eicosanoid lipoxin A4 is an agonist at FPR2/ALX. In comparison, fewer agonists have been identified for FPR3, the third member in this receptor family. Structural and functional studies of the FPRs have produced important information for understanding the general pharmacological principles governing all leukocyte chemoattractant receptors. This article aims to provide an overview of the discovery and pharmacological characterization of FPRs, to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature, and to discuss unmet challenges, including the mechanisms used by these receptors to bind diverse ligands and mediate different biological functions.

The oligopeptide transporter hPepT1: gateway to the innate immune response

Bacterial products that are normally present in the lumen of the colon, such as N-formylated peptides and muramyl-dipeptide, are important for inducing the development of mucosal inflammation. The intestinal dipeptide transporter, hPepT1, which is expressed in inflamed but not in noninflamed colonic epithelial cells, mediates the transport of these bacterial products into the cytosol of colonic epithelial cells. The small bacterial peptides subsequently induce an inflammatory response, including the induction of MHC class I molecules expression and cytokines secretion, via the activation of nucleotide-binding site and leucine-rich repeat (NBS-LRR) proteins, for example NOD2, and activation of NF-kappaB. Subsequent secretion of chemoattractants by colonic epithelial cells induces the movement of neutrophils through the underlying matrix, as well as across the epithelium. These bacterial products can also reach the lamina propria through the paracellular pathway and across the basolateral membrane of epithelial cells. As a consequence, small formylated peptides can interact directly with immune cells through specific membrane receptors. Since immune cells, including macrophages, also express hPepT1, they can transport small bacterial peptides into the cytosol where these may interact with the NBS-LRR family of intracellular receptors. As in intestinal epithelial cells, the presence of these small bacterial peptides in immune cells may trigger immune response activation.

G protein threshold behavior in the human neutrophil oxidant response: measurement of G proteins available for signaling in responding and nonresponding subpopulations

Threshold behavior is an important aspect of signal transduction pathways that allows for responses to be turned on or off. Human neutrophil responses to N-formyl peptides, including oxidant production and release, exhibit threshold behavior with respect to the number of G proteins available for signaling; progressive treatment of neutrophils with pertussis toxin causes the conversion of responding cells to nonresponding cells. To quantify the threshold level of G proteins required for signaling of N-formyl peptide stimulated oxidant production in a neutrophil population, we used a plasma membrane associated G protein quantification assay in conjunction with a sorting flow cytometer and measured differences in the average number of G proteins available for signaling per cell in both the responding and the nonresponding subpopulations after pertussis toxin treatment. Although there appeared to be a threshold separating responding cells and nonresponding cells for a given sample, no discrete threshold was measured across multiple treatment conditions. A mathematical model of the early steps in signaling suggests that cell-to-cell variability in signal parameters, such as numbers of signal components and values of kinetic rate constants, obscures the measurement of a discrete threshold and leads to an apparent decrease in the threshold level of G proteins available for signaling as the total G proteins are decreased.

hPepT1 transports muramyl dipeptide, activating NF-kappaB and stimulating IL-8 secretion in human colonic Caco2/bbe cells

BACKGROUND AND AIMS

Bacterial proteoglycan-derived muramyl dipeptide (MDP) activates the intracellular NOD2/CARD15 gene product. How intestinal epithelial cells take up MDP is poorly understood. We hypothesized that the intestinal apical di-/tripeptide transporter, hPepT1, transports MDP, thereby activating downstream pathways similar to nuclear factor kappa B (NF-kappaB).

METHODS

Time- and concentration-dependent (3)H-MDP uptakes were studied in Caco2/bbe (C2) cell monolayers where hPepT1 expression was either over- or underexpressed, using an inducible adenovirus system or silencing RNA (siRNA), respectively. NF-kappaB activation and interleukin (IL)-8 and monocyte chemoattractant protein-1 (MCP-1) release were determined by enzyme-linked immunosorbent assay. NOD2/CARD15 expression was inhibited by siRNA. MDP in human duodenal, cecal, and stool samples was measured.

RESULTS

MDP, but not its isoforms, inhibited uptake of glycosylsarcosine in C2 cells, indicating stereoselective and competitive inhibition. Approximately 90% of the MDP was cytosolic, showing uptake rather than binding. The K m for MDP uptake was 4.3 mmol/L. Cells overexpressing hPepT1 showed increased Gly-Sar and MDP uptake, whereas decreased uptake was observed after hPepT1 siRNA-inhibition. MDP treatment activated NF-kappaB, resulting in IL-8 release, an effect blocked by siRNA-inhibited expression of NOD2/CARD15. MDP content in cecal and stool samples (in normal subjects) was 20-87 micromol/L, but undetectable in duodenal fluid.

CONCLUSIONS

In colonic epithelial cells, MDP is taken up by hPepT1 and activates NF-kappaB and chemokine production. Because hPepT1 expression in chronic colonic inflammation is increased, this may play an important role in promoting colonocyte participation in host defense and pathogen clearance through increased uptake of MDP.

PepT1-mediated fMLP transport induces intestinal inflammation in vivo

In the present study, the effect of H(+)/peptide transporter (PepT1)-mediated N-formylmethionyl-leucyl-phenylalanine (fMLP) transport on inflammation in vivo in the rat small intestine, which expresses high PepT1 levels, and in the rat colon, which does not express PepT1, were investigated using myeloperoxidase (MPO) activity and histological analysis. We found that 10 microM fMLP perfusion in the jejunum for 4 h significantly increased MPO activity and altered the architecture of jejunal villi. In contrast, 10 microM fMLP perfusion in the colon for 4 h did not induce any inflammation. In addition, we have shown that 50 mM Gly-Gly alone did not affect basal MPO activity but completely inhibited the MPO activity induced by 10 microM fMLP in the jejunum. Together, these experiments demonstrate that 1) the differential expression of PepT1 between the small intestine and the colon plays an important role in epithelial-neutrophil interactions and 2) the inhibition of fMLP uptake by jejunal epithelial cells (expressing PepT1) reduces the neutrophil ability to move across the epithelium, in agreement with our previously published in vitro study. This report constitutes the first in vivo study showing the implication of a membrane transporter (PepT1) in intestinal inflammation.

Chemoattractant-stimulated NF-kappaB activation is dependent on the low molecular weight GTPase RhoA

Chemoattractants bind to seven transmembrane-spanning, G-protein-coupled receptors on monocytes and neutrophils and induce a variety of functional responses, including activation of the transcription factor NF-kappaB. The signaling mechanisms utilized by chemoattractants to activate NF-kappaB in human peripheral blood monocytes are poorly defined. We previously demonstrated that fMet-Leu-Phe (fMLP) stimulates NF-kappaB activation, and this function of fMLP requires phosphatidylinositol 3-kinase (PI3K). Here we present evidence that fMLP activates RhoA and that fMLP-induced NF-kappaB activation requires this small GTPase. Stimulation of monocytes with fMLP rapidly activated RhoA as well as NF-kappaB, and their activation was markedly reduced by pertussis toxin treatment. Pretreatment of monocyte with a RhoA inhibitor, C3 transferase from Clostridium botulinum, effectively blocked fMLP-induced NF-kappaB activation as well as interleukin-1beta gene expression. A dominant negative form of RhoA (T19N) also inhibited fMLP-stimulated reporter gene expression in a kappaB-dependent manner. Cotransfection of the monocytic THP1 cells with a constitutively active form of RhoA (Q63L) with the promoter reporter plasmid results in a marked increase in NF-kappaB-mediated reporter gene expression. Furthermore, the PI3K inhibitors wortmannin and LY294002 block RhoA activation induced by fMLP. These results demonstrate that low molecular weight GTPase RhoA is a novel signal transducer for fMLP-induced NF-kappaB activation and Galpha(i) or Galpha(o) class of heterotrimeric G proteins likely mediate RhoA activation via PI3K in human peripheral blood monocytes.

Colonic epithelial hPepT1 expression occurs in inflammatory bowel disease: transport of bacterial peptides influences expression of MHC class 1 molecules

BACKGROUND & AIMS

hPepT1 is an intestinal epithelial apical membrane transporter responsible for uptake of di/tripeptides (including bacterial derived proinflammatory n-formyl peptides). hPepT1 expression normally has a strict axial gradient-highest in the proximal small intestine with no expression in the colon.

METHODS

Small intestinal-like cells (Caco2-BBE), and colonic-like cells (HT29-Cl.19A), and colonic mucosa from diseased and control patients were used in the present study.

RESULTS

hPepT1 expression occurs aberrantly in the colon with chronic ulcerative colitis (6 patients) and Crohn's disease (4 patients), but not in normal colon (4 patients) or colon with microscopic colitis (4 patients). To model expression of hPepT1 by colonic-like cells in inflamed states, we stably transfected HT29-Cl.19A cells with a modified hPepT1 tagged on the N-terminus with green fluorescence protein. Analysis of transfected cells revealed that: GFP-hPepT1 protein, like the natural protein, is targeted to the apical plasma membrane. In addition, the tagged protein retains the capability of di/tripeptide absorption, and the expression of the tagged protein by HT29-Cl.19A cells permits absorption of N-formyl-methionyl-leucyl-phenylalanine (fMLP), as occurs in hPepT1 expressing Caco2-BBE cells. fMLP uptake by colonic cells expressing GFP-hPepT1 specifically enhances major histocompatibility complex class I surface expression.

CONCLUSIONS

These data collectively indicate that, in some states of chronic inflammation, hPepT1 may be anomolously expressed in the colon. Further, transport of fMLP by hPepT1 potentially stimulates expression of key accessory immune molecule, MHC-1.

Characterization of the priming effect by pituitary canine growth hormone on canine polymorphonuclear neutrophil granulocyte function

In this report, we demonstrate that canine growth hormone (cGH) is capable of priming canine polymorphonuclear neutrophil granulocytes (PMN) in a manner resembling that of human PMN. The cGH influences important functions that are involved in the process of recruitment of PMN, i.e., shape change, chemotaxis, CD11b/CD18 expression, adhesion, and subsequent transendothelial migration. Also, intracellular O(2)(-) production was evaluated. We investigated the priming effect by incubating PMN with purified pituitary cGH at various concentrations (10 to 800 microg/liter). The capacity for shape change was significantly (P < 0.05) enhanced, whereas the chemotactic response under agarose was significantly (P < 0.05) reduced. The chemotactic migration in Boyden chambers (10-microm-thick polycarbonate filter; lower surface count technique) was significantly (P < 0.05) enhanced, presumably due to cGH-induced hyperadhesiveness to the lower surface of the filters. The adhesion in albumin-coated microtiter plates and adherence to canine pulmonary fibroblasts were significantly (P < 0.05) increased, and the increased adhesion resulted in a significant (P < 0.01) increase in transendothelial migration using canine jugular vein endothelial cells. The increase in adhesion was associated with a significant increase in CD11b/CD18 expression. Furthermore, intracellular O(2)(-) production was significantly enhanced in response to both phorbol myristate acetate (P < 0.01) and opsonized zymosan (P < 0.05). In the absence of a PMN-stimulating agent, cGH did not influence the effector functions investigated except for an increased expression of CD11b/CD18.

Trp-Lys-Tyr-Met-Val-Met activates mitogen-activated protein kinase via a PI-3 kinase-mediated pathway independent of PKC

Trp-Lys-Tyr-Met-Val-Met (WKYMVM) is a novel potent peptide which can stimulate phosphoinositide hydrolysis in U937 as well as U266 and HL-60 cells (Baek et al., J. Biol. Chem. 271, 8170 (1996)). The peptide also induces superoxide generation in human neutrophils (Seo et al., J. Immunol. 158, 1896 (1997)). However, the signaling pathway down-stream of PLC set in motion by the peptide is not yet completely understood. We studied the signaling pathway of the peptide with the goal of elucidating the mechanism of the peptide's action. WKYMVM induced a rapid and transient activation of the ERKs in human histiocytic lymphoma cells, U937. The ERK1 activation peaked at 5 min and returned to the basal level after 30 min. The ERK1 stimulation by the peptide was partially inhibited by pretreatment of the cells with pertussis toxin (PTX), implicating G-protein involvement in the peptide's action. Pretreatment of staurosporine, protein kinase C (PKC) inhibitor, or PKC down-regulating PMA had no impact on the ERK1 activation by the peptide, indicating that the signaling pathway is independent of PKC activation. Pretreatment of the cells with neomycin and intracellular Ca2+ mobilizing reagents had also no effect on the ERK1 activation by the peptide. However, pretreatment with wortmannin or LY294002, the inhibitor of phosphatidylinositol 3 kinase (PI-3K), strongly inhibited peptide-stimulated ERK1 activation. Our results suggest that PI-3K may be an important participant in the ERK cascade induced by the peptide. Furthermore, the treatment of U937 cells with the peptide activated p74Raf-1, an upstream kinase of ERK. Taken together, our results suggest that the peptide activate ERK via a G-protein/PI-3K/Ras/Raf-1 mediated signaling pathway in U937 cells.

Protein kinase C antagonizes pertussis-toxin-sensitive coupling of the calcitonin receptor to adenylyl cyclase

The calcitonin receptor is known to couple to Gs and Gq, activating adenylyl cyclase and phospholipase C, respectively. The observation of pertussis-toxin-sensitive responses to calcitonin suggests that the receptor is capable of coupling to Gi/o as well. However, the calcitonin-dependent activation of adenylyl cyclase in HEK-293 cells that stably express the cloned rabbit calcitonin receptor, as in many other cells that express calcitonin receptors, shows little pertussis toxin sensitivity. Calcitonin treatment of these cells stimulates protein kinase C, which is reported to antagonize the receptor-dependent activation of Gi. The possibility that protein kinase C could be antagonizing Galphai-adenylyl cyclase coupling was tested by examining the effects of protein kinase C inhibitors (chelerythrine chloride and sphingosine) or of chronic treatment with phorbol ester to deplete protein kinase C. All three treatments led to a reduction of calcitonin-induced adenylyl cyclase activity that was reversed by pertussis toxin. Inhibiting or depleting protein kinase C had no effect on the activation of adenylyl cyclase by cholera toxin, indicating that Gs and adenylyl cyclase were not affected by these treatments. Calcitonin treatment of HEK-293 cells, that stably express a myc-tagged rabbit calcitonin receptor, induced the formation of complexes of the receptor and Galphai subunits, confirming that the calcitonin receptor interacts with Gi. Thus, the calcitonin receptor can couple to Gi, but the inhibition of adenylyl cyclase by Galphai is negatively regulated by protein kinase C.

Threshold and graded response behavior in human neutrophils: effect of varying G-protein or ligand concentrations

Observing the qualitative characteristics of response behavior as key variables in the signal transduction cascade are changed can provide insight into the fundamental roles of these interactions in producing cellular responses. Using flow cytometric assays and pertussis toxin (PT) treatment of human neutrophils, we have shown that actin polymerization stimulated with the chemoattractants N-formyl-Met-Leu-Phe, leukotriene B4, and interleukin-8 exhibits threshold behavior in terms of G-protein number. Partial PT treatment resulted in both responding and nonresponding populations of cells upon stimulation. As PT treatment was increased, the responding population of cells continued to respond maximally, while the number of cells responding decreased. We also showed that N-formyl peptide-stimulated oxidant production exhibits threshold behavior in terms of G-protein number, and the threshold for oxidant production is significantly greater than that for actin polymerization. The threshold behavior observed with PT treatment contrasted with the graded response behavior seen when cells were stimulated with different doses of ligand. For actin polymerization, only one population of cells was observed at submaximal ligand concentrations, and as ligand concentration was decreased the whole population responded submaximally. For oxidant production, as ligand concentration was decreased there were two populations of cells, but the responding cells responded submaximally. A mathematical model incorporating receptor/ligand binding and G-protein activation was developed to account for these differences in response behavior. Our results predict that an early signal transduction event in addition to, and not initiated by G-protein activation, is necessary to account for actin polymerization and oxidant production in neutrophils.

Activation of mitogen-activated protein kinases by formyl peptide receptors is regulated by the cytoplasmic tail

Wild type formyl peptide receptors (FPRwt) and receptors deleted of the carboxyl-terminal 45 amino acids (FPRdel) were stably expressed in undifferentiated HL-60 promyelocytes. Expression of FPRwt reconstituted N-formylmethionyl-leucyl-phenylalanine (FMLP)-stimulated extracellular signal-regulated kinase (ERK) and p38 kinase activity. Expression of FPRdel resulted in a 2-5-fold increase in basal ERK and p38 kinase activity, whereas FMLP failed to stimulate either mitogen-activated protein kinase (MAPK). Pertussis toxin abolished FMLP stimulation of both MAPKs in FPRwt cells but had no effect on either basal or FMLP-stimulated MAPK activity in FPRdel cells. FMLP stimulated a concentration-dependent increase in guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding in membranes from FPRwt but not FPRdel cells. GTPgammaS inhibited FMLP binding to FPRwt but not FPRdel membranes. Photoaffinity labeling with azidoanilide-[gamma-32P]GTP in the presence or absence of FMLP showed increased labeling only in FPRwt membranes. Immunoprecipitation of alphai2 and alphaq/11 from solubilized, photolabeled membranes showed that FPRwt were coupled to alphai2 but not to alphaq/11. FPRwt cells demonstrated calcium mobilization following stimulation with FMLP, whereas FPRdel cells showed no increase in intracellular calcium. We conclude that the carboxyl-terminal tail of FPRs is necessary for ligand-mediated activation of Gi proteins and MAPK cascades. Deletion of the carboxyl-terminal tail results in constitutive activation of ERK and p38 kinase through a Gi2-independent pathway.

Importance of the carboxy-terminus of the CXCR2 for signal transduction

The CXCR2 is phosphorylated at the C-terminal intracytoplasmic portion within 15 sec following the addition of IL-8 or MGSA. Cells transfected with a truncated form of the receptor missing the last 12 amino acids (T3) showed normal binding affinity, but were no longer phosphorylated; individual alanine replacement indicated that Ser346 and 348 were the primary sites of phosphorylation. In studies of the importance of phosphorylation in CXCR2 desensitization, cells expressing wild type CXCR2 lost GTP gamma S binding above basal rate after the first exposure to IL-8, while cells with the T3 mutant retained 60% of their capacity to induce GTP gamma S exchange upon a second exposure to IL-8. In contrast, receptor internalization was not affected by the loss of phosphorylation of the T3 mutant. Further receptor truncation led to decreasing binding affinities for IL-8 and MGSA and a decreased rate of GTP gamma S exchange following addition of excess ligand which suggests involvement of this region in G-protein coupling.

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.

Effect of cytochalasin B on intracellular free calcium concentration in human polymorphonuclear leukocytes after repeated stimulation with n-formyl-methionyl-leucyl-phenylalanine

Cytochalasin B can influence various functions of human polymorphonuclear leukocytes, including chemotaxis, lysosomal enzyme release, and reactive oxygen species generation. In this study we investigated the effect of cytochalasin B on the increase in intracellular free calcium concentration after three consecutive additions of 10(-7) M N-formyl-methionyl-leucyl-phenylalanine. The interval between stimulations was 5 min. Intracellular free calcium concentration was monitored using the fluorescent calcium indicator FURA-2AM. Cytochalasin B (3.3 micrograms/ml) added 60 s before the cell stimulation enhanced all three polymorphonuclear leukocyte calcium responses by increasing the N-formyl-methionyl-leucyl-phenylalanine-induced calcium influx from the extracellular space. Cytochalasin B increased the peak intracellular free calcium concentration and elevated the plateau phase level, but had no influence on its shape. In addition, pretreatment with cytochalasin B of polymorphonuclear leukocytes suspended in low calcium medium restored their capacity to respond to a third stimulation with N-formyl-methionyl-leucyl-phenylalanine. Finally, in resting cells cytochalasin B caused a moderate increase in intracellular free calcium concentration which was independent of extracellular calcium.

Binding of low affinity N-formyl peptide receptors to G protein. Characterization of a novel inactive receptor intermediate

G protein-coupled seven-transmembrane-containing receptors, such as the N-formyl peptide receptor (FPR) of neutrophils, likely undergo a conformational change upon binding of ligand, which enables the receptor to transmit a signal to G proteins. We have examined the functional significance of numerous conserved charged amino acid residues proposed to be located within or near the transmembrane domains. Whereas the wild type FPR exhibits a Kd for an agonist of 1-3 nM, which is reduced to approximately 40 nM in the presence of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S), substitution of either Asp71 or Arg123 resulted in mutant receptors that bound ligand with only low affinity (Kd = 30-50 nM) independent of GTP gamma S. In contrast, substitution of Arg163, predicted to be located at a similar depth within the membrane as Asp71, had no effect on ligand binding. Replacement of residues Arg309-Glu310-Arg311 resulted in an FPR with intermediate ligand binding characteristics. Functional analysis of the mutant receptors revealed that substitution of either Asp71 or Arg123 resulted in a mutant receptor that was unable to mediate calcium mobilization, whereas replacement of residues Arg309-Glu310-Arg311 yielded a receptor with an EC50 of 50 nM, compared with 0.5 nM for the wild type FPR. In order to determine the point of the defect in signal transduction, we performed reconstitution of the solubilized receptors with purified G proteins. The wild type FPR displayed a Kd for G protein of approximately 0.6 microM compared with the Arg309/Glu310/Arg311 mutant with a Kd of approximately 30 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemoattractant-induced NADPH oxidase activity in human monocytes is terminated without any association of receptor-ligand complex to cytoskeleton

When the chemotactic peptide formylmethionyl-leucyl-phenylalanine binds to its cell surface receptor, a transmembrane signal is generated that activates the superoxide-producing NADPH oxidase of human phagocytes. Comparing monocytes and neutrophils with regard to the production of superoxide anion induced by the peptide, we found a similar time-course for both types of cells. In neutrophils, ligand binding induced a conversion of the receptor to a high-affinity form, a change suggested to be due to an association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton. This event has been hypothesized to terminate the signal that activates the NADPH oxidase and thereby results in cessation of the cellular production of superoxide anion. Neutrophils preincubated with the cytoskeleton-disrupting drug cytochalasin B showed an increased and prolonged superoxide anion production after activation with the peptide, thus indicating that the cytoskeleton is involved in terminating this response. Formylmethionyl-leucyl-phenylalanine was also found to induce polymerization of actin in monocytes; however, cytochalasin B had no effect on the peptide-induced generation of superoxide anion in these cells. Furthermore, also in monocytes, ligand binding induced a conversion of the receptor to a high-affinity form; however, the receptor-ligand complex did not coisolate with the Triton X-100-insoluble cytoskeleton. These results indicate that, in monocytes, the NADPH oxidase activating pathway is terminated without any association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton.

Changes of intracellular free calcium concentration in human polymorphonuclear leukocytes after repeated stimulations with N-formyl-methionyl-leucyl-phenylalanine

A rapid transient rise in the intracellular free calcium concentration ( Ca2+]i) is an important step in human polymorphonuclear leukocytes (PMNL) activation. This can be caused by many inflammatory mediators and has been implicated in the regulation of various cellular reactions. In this study we investigated the changes of [Ca2+]i in human PMNL activated three times with 10(-7)M n-formyl-methionyl-leucyl-phenylalanine (FMLP). PMNL in the presence of 1 mM Ca2+ were able to respond to three consecutive stimulations with FMLP. The first Ca2+ response was the highest one and was a result of Ca2+ release from internal stores (which was responsible for about 30% of maximal increment in [Ca2+]i) and the extracellular Ca2+ influx. Experiments with PMNL suspended in a medium containing 100 nM Ca2+ and pretreated with 1 nM Ni2+ (an inorganic calcium channel blocker) revealed that the second and third response is completely dependent on the extracellular Ca2+ influx. Changes of the time interval between stimulations had no influence on the occurrence of extracellular Ca2+ influx related to second addition of FMLP. Elongation of the time interval up to 30 min did not restore the release of Ca2+ from internal stores. It indicates the occurrence of dissociation of Ca2+ release from intracellular stores and extracellular Ca2+ influx during the second and third PMNL response to FMLP.

Characterization of complex formation between Gi2 and octyl glucoside solubilized neutrophil N-formyl peptide chemoattractant receptor by sedimentation velocity

The reversible formation of complexes between N-formyl peptide chemoattractant receptor (FPR) and Gi2 protein was analyzed by velocity sedimentation in linear sucrose density gradients. FPR complexed with heterotrimeric Gi2, sediments at different rate than uncomplexed FPR and the two forms have apparent sedimentation coefficients of 7S and 4S, respectively. The biochemical variables important for the reconstitution of the 7S complex from the 4S receptor and Gi2 were studied. The formation of 7S was saturable with Gi2 and addition of excess Gi did not cause oligomerization. The reconstituted 7S complex was stable under a variety of conditions including octyl glucoside concentrations below and above the critical micellar concentration. The optimum pH for the reconstitution is between 7 and 9, where the 4S and 7S species sedimented reproducibly, at distinct positions in the gradient. Below pH 6 both the 4S and the 7S species appear to undergo denaturation and form precipitates. Magnesium ions have no significant effect on the sedimentation of either forms of FPR. Reconstitution was stable up to a NaCl concentration of 0.2 M. At 1 M NaCl reconstitution was inhibited and at 3 M salt FPR aggregated. Since guanine nucleotides GTP, GTP gamma S, GDP beta S selectively dissociated the 7S complex in a concentration-dependent manner and adenine nucleotides had no effect, we conclude that the FPR-Gi2 system displays a vacant guanyl nucleotide binding site, the hallmark of a functional guanine nucleotide exchange complex. Moreover, our results indicate that the reconstitution of FPR-Gi2 complexes is reproducible at physiologically relevant conditions, shows selectivity, specificity, and biochemically functional properties consistent with a specific and functional interaction between solubilized FPR and G protein.

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.

Neutrophil chemoattractant receptors and the membrane skeleton

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal-elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin.

Cytoskeletal regulation of chemotactic receptors: molecular complexation of N-formyl peptide receptors with G proteins and actin

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process. It involves direct interaction of receptors with heterotrimeric G-proteins and may be under the control of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton determines the distribution of FPR in the plane of the plasma membrane, thus controlling FPR accessibility to different proteins in functionally distinct membrane domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. We are investigating the molecular basis for the interaction of FPR with the membrane skeleton, and our results suggest that FPR, and possibly other receptors, may directly bind to cytoskeletal proteins such as actin.