Immunochemical characterization of the formyl peptide receptor moieties on human neutrophils.

The neutrophil (PMN) receptor for formylated peptides such as N-formyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP) is involved in binding and subsequent response to certain chemotactic stimuli. The receptor on human PMN has been reported to consist of several glycoprotein components, ranging in size from 43-94 kDa. Furthermore, FMLP receptors on human PMN have been shown to contain both high and low affinity states. In this study, the receptor was purified by subjecting solubilized PMN plasma membrane components to FMLP-affinity chromatography, and was found to be comprised of four components, one of 68 kDa, and the others of 94, 48, and approximately 40 kDa. Only the 68, the 94, and the approximately 40 kDa components specifically bound a radioiodinated FMLP analogue. To further characterize these components, a battery of monoclonal antibodies reactive against the FMLP receptor was prepared. Seven monoclonal antibodies were selected on the basis of their reactivity with the 68 kDa receptor component. Some of these antibodies also cross-react with the 48 kDa component, suggesting that the 68 and the 48 kDa receptor moieties are immunologically related. These antibodies reacted with normal human neutrophils, but not with lymphocytes, or unstimulated HL-60 cells. Furthermore, the presence of 20 nmol of FMLP inhibited the binding of five of the anti-receptor antibodies to whole PMN. These results suggest that the epitopes recognized by these five antibodies may possibly be involved in FMLP binding.     

House dust mite allergen activates human eosinophils via formyl peptide receptor and formyl peptide receptor-like 1

The objective was to evaluate which receptors house dust mite (HDM) and birch pollen extracts engage to activate human eosinophils. Chemotaxis and degranulation were studied in eosinophils pretreated with pertussis toxin and other antagonists of G protein-coupled receptors, e.g. the formyl peptide receptor (FPR), CC chemokine receptor 3 (CCR3) and leukotriene receptor B4 (LTB(4)R). Inhibition of the FPR as well as desensitization of the receptor rendered eosinophils anergic to activation by the allergens. Blockade of CCR3 or LTB(4)R did not affect eosinophilic reactivity. It was determined by PCR that human eosinophils express the FPR family members FPR and FPR-like 1 (FPRL1). HDM, unlike birch pollen, evoked calcium fluxes in HL-60 cells transfected with FPR or FPRL1. Although both allergens gave rise to calcium transients in neutrophils, which also express FPR and FPRL1, only the HDM response was decreased by the FPR antagonist. Moreover, neutrophils migrated toward HDM but not to birch pollen. Eosinophils pretreated with inhibitors of MAPK p38, ERK1/2 or protein kinase C exhibited diminished responsiveness to the aeroallergens. This study indicates that FPR and FPRL1 mediate the activation of eosinophils by HDM, whereas birch pollen employs other pathways shared with FPR to activate human eosinophils.     

C5a and formyl peptide receptor regulation on human monocytes.

Regulation of C5a and formyl-methionine-leucine-phenylalanine-lysine (fMLPL) receptors on human monocytes has been studied using fluorescein-conjugated derivatives and flow cytometry. Monocytes have receptors for each of these ligands, as evidenced by their ability to bind specifically biologically active fluorescein derivatives of these ligands. Quenching experiments showed that bound fluoresceinated C5a and fMLPL are rapidly internalized at 37 degrees C. Once internalized, monocytes are able to reexpress these receptors, returning to control levels within approximately 90 min. This contrasts with rate differences seen in polymorphonuclear neutrophils (PMNs), where fMLPL receptors return more rapidly (approximately 30 min) than do C5a receptors (approximately 100 min). Monensin inhibited the reexpression of C5a but not fMLPL receptors, suggesting that a receptor recycling process is necessary to replenish C5a receptors on the monocyte surface. Similar although less efficient inhibition of C5a receptor reexpression was observed with NH4Cl treatment. Reexpression of both C5a and fMLPL receptors was independent of extracellular Ca2+. Treatment with various agents known to stimulate monocytes and PMNs increased the expression of fMLPL receptors in both cell types but either had no effect on or reduced the level of C5a receptor expression. This would indicate that monocytes, like PMNs, have intracellular pools of preformed fMLPL receptors, available for reexpression. These studies show that, like PMNs, monocytes modulate C5a and fMLPL receptors through different mechanisms. Furthermore, monocytes are capable of reexpressing these receptors following exposure to ligand, a theoretical requirement for chemotaxis.     

Characterization of the rat neutrophil formyl peptide chemotaxis receptor.

Numerous synthetic N-formylated peptides, believed to be the analogs of the naturally occurring initiating signal peptides produced by bacteria, are potent chemotactic agents for phagocytic cells in several species. The authors have characterized the receptor with moderately high affinity for the chemotactic peptide f-Met-Leu-[3H]Phe on the rat peritoneal neutrophils. When neutrophils are incubated with f-Met-Leu-[3H]Phe at 24 C, the binding is saturable and reversible. The receptor on the inflammatory rat neutrophils has an equilibrium dissociation constant (KD) of 3.4 x 10(-8) M at 24 C, and there are approximately 65,000 sites per cell. In addition, the potency of several of these chemotactic peptides in inducing lysosomal enzyme secretion and superoxide production correlated well with their ability to compete with f-Met-Leu-[3H]Phe for receptor binding. Structure activity studies further demonstrate that the fine specificity of the formyl peptide receptor has been conserved across species lines.     

甲酰肽受体家族受体与恶性肿瘤

甲酰肽受体家族受体包括甲酰肽受体（FPR）、类甲酰肽受体1和类甲酰肽受体2是主要表达在吞噬细胞表面的一类七次跨膜、G蛋白偶联受体。这些受体被相应的激动剂激活后，能活化吞噬细胞，引起细胞的趋化移动和炎症介质的释放等生物学效应，从而在天然免疫和炎症反应中发挥重要作用。近年来，人们发现甲酰肽受体家族受体也高表达于某些恶性肿瘤细胞的表面，且与这些肿瘤的发生、发展、转移以及治疗有着密切的关系，这无疑揭示了甲酰肽受体家族受体一个新的重要作用，值得深入研究和探讨。     

Production and characterization of a recombinant ligand-binding domain of the human low-density lipoprotein receptor

Two plasmids carrying the cDNA fragment of human low-density lipoprotein receptor coding for its ligand-binding domain are constructed. The ligand-binding domain of the receptor composed of fused and individual polypeptides is purified and characterized. Translated fromByulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 123, No. 2, pp. 184–187, February, 1997     

The chemoattractant Trp-Lys-Tyr-Met-Val-D-Met activates eosinophils through the formyl peptide receptor and one of its homologues,formyl peptide receptor-like 1

Whereas prokaryotes use L- and D-isomers of amino acids in their protein synthesis, eukaryotic proteins as a rule incorporate only L-isomers. Hence, D-isomers may constitute danger signals to the innate immune system. A D-methionine-containing peptide, Trp-Lys-Tyr-Met-Val-D-Met-NH(2) (WKYMVm), has been shown to be a stronger activator of neutrophils than f-Met-Leu-Phe. The aim of this study was to compare the responsiveness of eosinophils to WKYMVm with that of neutrophils. The peptide was found to induce chemotaxis and respiratory burst in eosinophils. However, it did not mobilize granule constituents, as evidenced by a lack of eosinophil cationic protein, eosinophil peroxidase, and interleukin-5 in the supernatants of stimulated eosinophils. In contrast, WKYMVm caused the release of complement receptor 3 from secretory vesicles in neutrophils. Different members of the formyl peptide receptor family were preferentially engaged by the peptide in the two classes of granulocytes: the formyl peptide receptor itself in eosinophils and formyl peptide receptor-like 1 in neutrophils.     

Structural similarities between mRNA for the formyl peptide receptors and 18S rRNA

Formyl peptides released by some bacteria are powerful chemoattractants and activators of mammalian granulocytes and monocytes, acting through 7-transmembrane specific formyl peptide receptors (FPRs). Three distinct segments of the formyl peptide receptor 1 (FPR1) mRNA of Man share probabilistically significant homologies with segments of the 18S rRNA which are highly conserved from Drosophila to Man. Overall, the three segments cover ≈ 24% that of the 18S rRNA sequence and ≈ 36% of the FPR1 sequence. The three segments are, however, arranged in different orders in the 18S rRNAs and in the FPR1 mRNA, the segment appearing in the first location in the 18S rRNAs is located at the end of the FPR1 mRNA sequence. The hypothesis is advanced that the three “conserved” segments either derive from an ancestral gene that is the forerunner of both the ribosomal 18S genes and the FPR genes or that at some stage of evolution the FPR genes derived, at least in part, from the more ancient ribosomal 18S genes. The extant 18S rRNA sequences exhibit obvious signs of a number of breaks that occurred during evolution, especially in the transition from insects to vertebrates. Some of these events may have resulted in differential rearrangements of segments in the groups of FPR genes and ribosomal 18S genes.     

Molecular biology for formyl peptide receptors in human diseases

Leukocytes accumulate at sites of inflammation and immunological reaction in response to locally existing chemotactic mediators. The first chemotactic factors structurally defined were N-formyl peptides. Subsequently, numerous ligands were identified to activate formyl peptide receptors (FPRs) that belong to the seven-transmembrane G protein-coupled receptor superfamily. FPRs interact with this menagerie of structurally diverse pro- and anti-inflammatory ligands to possess important regulatory effects in multiple diseases, including inflammation, amyloidosis, Alzheimer’s disease, prion disease, acquired immunodeficiency syndrome, obesity, diabetes, and cancer. How these receptors recognize diverse ligands and how they contribute to disease pathogenesis and host defense are basic questions currently under investigation that would open up new avenues for the future management of inflammation-related diseases.     

Microfluidic devices for characterizing the agonist of formyl peptide receptor in RBL-FPR cells

The human formyl peptide receptor (FPR) plays an important role in inflammation and immunity. Finding of specific agonists and antagonists of FPR may provide potential therapeutic agents for FPR related disorders. The binding of agonist by FPR induces a cascade of G protein-mediated signaling events leading to neutrophil chemotaxis, intracellualr calcium mobilization, FPR ligand uptake and so on. This work proposed a microfluidic-based method to characterize FPR-related cellular events in response to small peptides, N-formyl-Met-Leu-Phe (fMLF), in rat basophilic leukemia cell line RBL-2H3 expressing human FPR (RBL-FPR). The results showed that fMLF triggered chemotaxis, calcium mobilization and FPR ligand uptake in RBL-FPR cells, indicating the potential role of FPR agonist. The chemotaxis index and the calcium mobilization intensity increased but the time course of calcium mobilization decreased, as the rising of fMLF concentration. The basic agreement between the microfluidic results and the previous studies demonstrated good feasibility of the microfluidic method for characterization of FPR agonist. Microfluidic technology displays significant advantages over traditional methods in terms of sample consumption and assay time. It also facilitates experimental process and real-time observation of cellular responses at single cell resolution.     

Structural relatedness between the 18S rRNA genes and the formyl peptide receptor genes: new insights into the phylogenesis of immune receptors

In this study the authors examined the sequences of the ribosomal 18S rRNA of Drosophila and man and 16 mRNA sequences coding for different members of the family of the mammalian formyl peptide receptors (FPRs). The positions in the sequences of all >or=7-base oligonucleotide identities occurring in at least one of the 18S rRNAs and one of the FPR mRNAs were recorded. On the basis of the positional data, the Drosophila 18S-FPR and human 18S-FPR distances (in nucleotides) were determined for each identity. Then the actual frequency distribution of the distances (grouped into 200-unit classes) was derived. The theoretical frequency distribution of distances was also calculated under the assumption of non-relatedness between the 18S and FPR sequences. Comparison between the theoretical and the actual distributions showed that at class -500 (range from - 400 to - 600) of the 18S-FPR values the actual frequency was significantly (p < 0.01) higher than the theoretical frequency, in both Drosophila and man, suggesting that the second section of the FPR genes (approximately from nucleotide 400 to the end of sequence) may be structurally related to the first section of the ribosomal 18S genes (approximately nucelotides 1-650). The authors advance the hypothesis that the two families of genes may have used common ancestral raw genetic materials in the building of the extant sequences.     

Structural and functional characterization of a human IgG monoclonal antiphospholipid antibody

Antiphospholipid antibodies (aPL) are likely involved in the pathogenesis of the antiphospholipid syndrome (APS). This study analyzes the structural and functional characteristics of a human monoclonal aPL (HL7G) from the IgG2 subtype with λ light chains generated from a patient with primary APS and recurrent cerebral microemboli. DNA encoding the variable region of heavy and light chains of the antibody was sequenced, analyzed, and compared to HL5B a previously described monoclonal aPL from the same patient. Both antibodies are derived from the same germline genes. HL7G had similar but more extensive somatic mutations in the CDR1 and 2 regions than HL5B, indicating that both antibodies are closely related and derived by a T cell-dependent antigen driven process. In ELISA assays HL7G bound to cardiolipin and several other phospholipid antigens in the absence of protein cofactors. Different from HL5B this aPL bound to β2-glycoprotein I (β2GPII). This suggests that reactivity of aPL against β2GPI is determined by only few specific amino acid exchanges. HL7G was able to induce tissue factor (TF) as one of the procoagulant effects of aPL. Our data suggest that the binding specificity of aPL is only of limited value to predict the biological effect and the pathophysiological impact of the antibodies.     

Annexin 1 released by necrotic human glioblastoma cells stimulates tumor cell growth through the formyl peptide receptor 1

Highly malignant human gliomas overexpress the G-protein-coupled chemoattractant receptor formyl peptide receptor (FPR1), which promotes tumor progression when activated. Our previous studies demonstrated that necrotic glioblastoma cells release chemotactic agonist(s) that activate FPR1 on viable tumor cells. In the present study, we identified an FPR1 agonist released by necrotic human glioblastoma cells. Necrotic tumor cell supernatant (NecSup) contained Annexin 1 (Anx A1), a chemotatic polypeptide agonist for FPR1. Immunoabsorption of Anx A1 with a specific antibody markedly reduced the chemotactic activity of NecSup for tumor cells and diminished its capacity to promote tumor cell growth, invasion, and colony formation on soft agar. In addition, Anx A1 was present in tumor xenografts formed by human glioblastoma cells in nude mice. Anx A1 knockdown significantly reduced the tumorigenicity of glioblastoma cells in nude mice, but FPR1/Anx A1 double knockdown diminished tumor growth even further. The clinical relevance of Anx A1 in gliomas was supported by the observation that Anx A1 was more highly expressed in poorly differentiated human primary gliomas compared with lower grade tumors. Our study implicates Anx A1 as a major component in necrotic tumor cell-derived stimulants of the growth of glioblastoma via the activation of FPR1.     

Structural characterization of the main immunogenic region of the Torpedo acetylcholine receptor

To develop antigen-specific immunotherapies for autoimmune diseases, knowledge of the molecular structure of targeted immunological hotspots will guide the production of reagents to inhibit and halt production of antigen specific attack agents. To this end we have identified three noncontiguous segments of the Torpedo nicotinic acetylcholine receptor (AChR) α-subunit that contribute to the conformationally sensitive immunological hotspot on the AChR termed the main immunogenic region (MIR): α(1–12), α(65–79), and α(110–115). This region is the target of greater than 50% of the anti-AChR Abs in serum from patients with myasthenia gravis (MG) and animals with experimental autoimmune myasthenia gravis (EAMG). Many monoclonal antibodies (mAbs) raised in one species against an electric organ AChR cross react with the neuromuscular AChR MIR in several species. Probing the Torpedo AChR α-subunit with mAb 132A, a disease inducing anti-MIR mAb raised against the Torpedo AChR, we have determined that two of the three MIR segments, α(1–12) and α(65–79), form a complex providing the signature components recognized by mAb 132A. These two segments straddle a third, α(110–115), that seems not to contribute specific side chains for 132A recognition, but is necessary for optimum antibody binding. This third segment appears to form a foundation upon which the three-dimensional 132A epitope is anchored.     

Serum amyloid A mediates human neutrophil production of reactive oxygen species through a receptor independent of formyl peptide receptor like-1

Serum amyloid A (SAA) is one of the acute-phase reactants, a group of plasma proteins that increases immensely in concentration during microbial infections and inflammatory conditions, and a close relationship between SAA levels and disease activity in rheumatoid arthritis (RA) has been observed. RA is an inflammatory disease, where neutrophils play important roles, and SAA is thought to participate in the inflammatory reaction by being a neutrophil chemoattractant and inducer of proinflammatory cytokines. The biological effects of SAA are reportedly mediated mainly through formyl peptide receptor like-1 (FPRL1), a G protein-coupled receptor (GPCR) belonging to the formyl peptide receptor family. Here, we confirmed the affinity of SAA for FPRL1 by showing that stably transfected HL-60 cells expressing FPRL1 were activated by SAA and that the response was inhibited by the use of the FPRL1-specific antagonist WRWWWW (WRW4). We also show that SAA activates the neutrophil NADPH-oxidase and that a reserve pool of receptors is present in storage organelles mobilized by priming agents such as TNF-alpha and LPS from Gram-negative bacteria. The induced activity was inhibited by pertussis toxin, indicating the involvement of a GPCR. However, based on FPRL1-specific desensitization and use of FPRL1 antagonist WRW4, we found the SAA-mediated effects in neutrophils to be independent of FPRL1. Based on these findings, we conclude that SAA signaling in neutrophils is mediated through a GPCR, distinct from FPRL1. Future identification and characterization of the SAA receptor could lead to development of novel, therapeutic targets for treatment of RA.     

Redox signaling regulates commensal-mediated mucosal homeostasis and restitution and requires formyl peptide receptor 1

The mammalian gut microbiota is essential for normal intestinal development, renewal, and repair. Injury to the intestinal mucosa can occur with infection, surgical trauma, and in idiopathic inflammatory bowel disease. Repair of mucosal injury, termed restitution, as well as restoration of intestinal homeostasis involves induced and coordinated proliferation and migration of intestinal epithelial cells. N-formyl peptide receptors (FPRs) are widely expressed pattern recognition receptors that can specifically bind and induce responses to host-derived and bacterial peptides and small molecules. Here we report that specific members of the gut microbiota stimulate FPR1 on intestinal epithelial cells to generate reactive oxygen species via enterocyte NADPH oxidase 1 (NOX1), causing rapid phosphorylation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase mitogen-activated protein kinase. These events stimulate migration and proliferation of enterocytes adjacent to colonic wounds. Taken together, these findings identify a novel role of FPR1 as pattern recognition receptors for perceiving the enteric microbiota that promotes repair of mucosal wounds via generation of reactive oxygen species from the enterocyte NOX1.