Impaired antibacterial host defense in mice lacking the N-formylpeptide receptor

Nordy, a synthetic lipoxygenase inhibitor, inhibits the expression of formylpeptide receptor and induces differentiation of malignant glioma cells

We recently found that formylpeptide receptor (FPR), a G-protein-coupled receptor that mediates chemotaxis of phagocytic leukocytes induced by bacterial peptide N-formyl-methionyl-leucyl-phenylalanine, is expressed by malignant human glioma cells and promotes tumor growth and angiogenesis. In this study, we examined the effect of Nordy, a novel chiral lipoxygenase inhibitor which was synthesized based on the structure of a natural nordihydroguaiaretic acid, on the expression of FPR by human glioblastoma cells. We found that FPR was expressed at the protein level by highly malignant human glioma cell lines U87 and BT325, and a rat glioma cell line C6. The expression level of FPR was correlated with the degree of the malignancy of tumor cells. The poorly differentiated glioma cell line U87 expressed the highest level of FPR. In U87 glioma cells, the expression of FPR was attenuated at the protein level by Nordy treatment for 48 (P<0.05). Nordy did not affect FPR mRNA expression in U87 cells. In addition, Nordy treatment seemed to promote glioma cell differentiation, as evidenced by their reduced expression of vimentin and increased expression of GFAP. Our results suggest that Nordy was capable of reducing the level of malignancy of glioma cells.

The merits of in vitro versus in vivo modeling in investigation of the immune system

Immunity is vital for determining self and for the recognition and swift eradication of foreign antigens without harming the host. Innate immunity developed in metazoan, multi-cellular organisms under overwhelming selection pressure of invasive microbes and, although imperfect, has performed admirably to enable the evolution of higher eukaryotes. Adaptive immunity developed within an existing innate immune system to more effectively eradicate foreign antigens, whether from pathogens, malignant cells, or microbial toxins, such that repeated stimulations with foreign antigens are more efficiently excluded. Investigation of the immune system requires both in vivo and in vitro experimentation, not only because of the inherent complexity of immunity and the required pertinence of using higher mammals to not falsely disrupt the immune system, but also to use isolates of the specific cellular and humoral components to determine function, signal transduction, and a possible role of these constituents without the complexity and redundancy of immunity in intact animals. The hypotheses of well-designed in vitro experiments must also be tested in intact in vivo models to determine relevance and to discard artifactual findings secondary to the in vitro environment. The following review outlines the basic constituents and functions of both adaptive and innate immunity to demonstrate the importance of both in vivo and in vitro investigation of immunity in our attempt to define host defense and to decrease morbidity and mortality in humans.

Neutrophil chemotaxis by pathogen-associated molecular patterns - formylated peptides are crucial but not the sole neutrophil attractants produced by Staphylococcus aureus

The chemotactic migration of phagocytes to sites of infection, guided by gradients of microbial molecules, plays a key role in the first line of host defence. Bacteria are distinguished from eukaryotes by initiation of protein synthesis with formyl methionine. Synthetic formylated peptides (FPs) have been shown to be chemotactic for phagocytes, leading to the concept of FPs as pathogen-associated molecular patterns (PAMPs). However, it remains unclear whether FPs are major chemoattractants released by bacteria and whether further chemoattractants are produced. A Staphylococcus aureus mutant whose formyltransferase gene was inactivated (Deltafmt) produced no FPs and the in vitro and in vivo ability of Deltafmt culture supernatants to recruit neutrophils was considerably reduced compared with those of the parental strain. However, some chemotactic activity was retained, indicating that bacteria produce also unknown, non-FP chemoattractants. The activity of these novel PAMPs was sensitive to pertussis toxin but insensitive to the formyl peptide receptor inhibitor CHIPS. Deltafmt culture supernatants caused reduced calcium ion fluxes and reduced CD11b upregulation in neutrophils compared with wild-type supernatants. These data demonstrate an important role of FPs in innate immunity against bacterial infections and indicate that host chemotaxis receptors recognize a larger set of bacterial molecules than previously thought.

The neuropeptide pituitary adenylate cyclase activating protein is a physiological activator of human monocytes

Pituitary adenylate cyclase activating protein (PACAP) and its structurally related vasointestinal peptide (VIP) bind to three G-protein-coupled receptors named VPAC1 and VPAC2 for VIP/PACAP receptors and PAC1 for PACAP preferred receptors. We report that in freshly isolated human monocytes PACAP acts as a pro-inflammatory molecule. By RT-PCR, VPAC1 mRNA was the only receptor found to be expressed; VPAC1 protein was detected by Western blotting and visualized by immunohistochemistry. Signaling pathways activated by PACAP include the extracellular regulated kinase (ERK), the stress-activated MAPK p38, the focal adhesion kinase, Pyk2 and its associated cytoskeleton protein paxillin and the phosphatidylinositol 3-kinase (PI-3K). PACAP induces a transient peak in cytoplasmic calcium associated with an increase in reactive oxygen species production and upregulation in membrane expression of the integrin CD11b as well as the complement receptor 1. Control of the different pathways and functions stimulated by PACAP were evaluated using Phospholipase C (PLC), PI-3K, ERK and p38 MAPK inhibitors and led to the conclusion that PLC and to a lesser degree PI-3K activation are upstream events occurring in VPAC1 mediated PACAP stimulation of monocytes and are in contrast to ERK and p38 mandatory for the initiation of other cellular events associated with monocytes activation.

Human mitochondria-derived N-formylated peptides are novel agonists equally active on FPR and FPRL1, while Listeria monocytogenes-derived peptides preferentially activate FPR

N-formyl peptides are cleavage products of bacterial and mitochondrial proteins, and can attract leukocytes to sites of infection or tissue damage. In this study, HL-60 cell lines expressing the human N-formyl peptide receptor FPR or its two homologues (FPRL1, FPRL2) were used to determine the receptor selectivity of N-formylated peptides derived from Listeria monocytogenes or from human mitochondrial proteins. Bacterial peptides were 100-fold more potent on FPR than on FPRL1, whereas none of them could trigger intracellular signaling through FPRL2. In contrast, N-formylated hexapeptides corresponding to the N terminus of mitochondrial NADH dehydrogenase subunits 4 (fMLKLIV) and 6 (fMMYALF), and cytochrome c oxidase subunit I (fMFADRW) were equally potent on FPR and FPRL1. They triggered cellular responses with the following order of potency: fMMYALF > fMLKLIV > fMFADRW, with an EC50, in a Fura-2 calcium mobilization assay, of 10 nM, 44 nM, and 160 nM on FPR-expressing cells, and 15 nM, 55 nM and 120 nM on FPRL1-expressing cells. fMMYALF was also a low-affinity agonist of FPRL2 (EC50 of 1 microM) and was chemotactic for both FPRL1- and FPRL2-expressing cells. We identified novel mitochondrial host-derived agonists for human N-formyl-peptide receptors that might play a role in inflammatory or degenerative processes linked to their stimulation.

The immunostimulatory peptide WKYMVm-NH activates bone marrow mouse neutrophils via multiple signal transduction pathways

G-protein-coupled receptors play a major role in the activation of the innate immune system, such as polymorphonuclear neutrophils. Members of the formyl peptide receptor family recognize chemotactic peptides as well the amyloïd-beta peptide and fragments of the human immunodeficiency virus envelope and may thus be implicated in major pathologies. The peptide WKYMVm-NH2 probably activates the receptor FPRL1 and its mouse orthologues Fpr-rs1 and Fpr-rs2. We examined the stimulation of C57BL6 mouse neutrophils by WKYMVm-NH2 and the effects of several inhibitors for intracellular signalling pathways (wortmannin, LY 294002, staurosporin, H-89, U 73122, thapsigargin and SKF 96365). We show here that WKYMVm-NH2 is a powerful stimulator of primary and secondary granule exocytosis as well as superoxide production. The signalling pathway involves phosphoinositide 3-kinase, protein kinase C, phospholipase C and store-operated calcium influx. Studies with peptide antagonists suggest that WKYMVm-NH2 preferentially activates exocytosis via FPRL1 and not FPR, the major receptor for N-formylated peptides such as fMLF. However, the signalling pathways activated by WKYMVm-NH2 in mouse neutrophils are similar to those activated by fMLF in human neutrophils. Thus, the effect and the signalling pathways of the two agonists and their receptors are at least partially overlapping.

A twist in anti-inflammation: annexin 1 acts via the lipoxin A4 receptor

The inflammatory response is a life-saving protective process mounted by the body to overcome pathogen infection and injury; however, in chronic inflammatory pathologies this response can become deregulated. The existence of specialized anti-inflammatory pathways/mediators that operate in the body to down-regulate inflammation have now emerged. Thus, persistence of inflammation leading to pathology could be due to malfunctioning of one or more of these counter-regulatory pathways. Here we focus on one of them, the anti-inflammatory mediator annexin 1, and provide an update on its inhibitory effects upon the leukocyte trafficking process. In particular, recent evidence that receptors of the formyl-peptide family, which includes also the lipoxin A4 receptor, could be the annexin 1 receptor(s) in the context of anti-inflammation might provide new avenues for exploiting this pathway for drug discovery.

Integration of virtual screening with high-throughput flow cytometry to identify novel small molecule formylpeptide receptor antagonists

The formylpeptide receptor (FPR) family of G-protein-coupled receptors contributes to the localization and activation of tissue-damaging leukocytes at sites of chronic inflammation. We developed a FPR homology model and pharmacophore (based on the bovine rhodopsin crystal structure and known FPR ligands, respectively) for in silico screening of approximately 480,000 drug-like small molecules. A subset of 4324 compounds that matched the pharmacophore was then physically screened with the HyperCyt flow cytometry platform in high-throughput, no-wash assays that directly measure human FPR binding, with samples (each approximately 2500 cells in 2 microl) analyzed at 40/min. From 52 confirmed hits (1.2% hit rate), we identified 30 potential lead compounds (inhibition constant, Ki= 1-32 microM) representing nine distinct chemical families. Four compounds in one family were weak partial agonists. All others were antagonists. This virtual screening approach improved the physical screening hit rate by 12-fold (versus 0.1% hit-rate in a random compound collection), providing an efficient process for identifying small molecule antagonists.

Mouse cathelin-related antimicrobial peptide chemoattracts leukocytes using formyl peptide receptor-like 1/mouse formyl peptide receptor-like 2 as the receptor and acts as an immune adjuvant

Mammalian antimicrobial proteins, such as defensins and cathelicidin, have stimulating effects on host leukocytes. Cathelin-related antimicrobial peptide (CRAMP), the orthologue of human cathelicidin/LL-37, is the sole identified murine cathelicidin. CRAMP has been shown to have both antimicrobial and angiogenic activities. However, whether CRAMP, like human cathelicidin/LL-37, also exhibits a direct effect on the migration and function of leukocytes is not known. We have observed that CRAMP, like LL-37, was chemotactic for human monocytes, neutrophils, macrophages, and mouse peripheral blood leukocytes. CRAMP also induced calcium mobilization and the activation of MAPK in monocytes. CRAMP-induced calcium flux in monocytes was desensitized by MMK-1, an agonistic ligand specific for formyl peptide receptor-like-1 (FPRL1), and vice versa, suggesting the use of FPRL1 by CRAMP as a receptor. Furthermore, CRAMP induced the chemotaxis of human embryonic kidney 293 cells transfected with either FPRL1 or mouse formyl peptide receptor-2, the mouse homologue of FPRL1, but not by untransfected parental human embryonic kidney 293 cells, confirming the use of FPRL1/mouse formyl peptide receptor-2 by CRAMP. Injection of CRAMP into mouse air pouches resulted in the recruitment predominantly of neutrophils and monocytes, indicating that CRAMP acts as a chemotactic factor in vivo. Finally, simultaneous administration of OVA with CRAMP to mice promoted both humoral and cellular Ag-specific immune responses. Thus, CRAMP functions as both a chemoattractant for phagocytic leukocytes and an enhancer of adaptive immune response.

Formylpeptide Receptor FPR and the Rapid Growth of Malignant Human Gliomas

BACKGROUND

The formylpeptide receptor (FPR) is a G-protein-coupled receptor (GPCR) that mediates chemotaxis of phagocytic leukocytes induced by bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLF). We previously showed that selected human glioma cell lines also express functional FPR. We therefore investigated the relationship between FPR expression and the biologic behavior of glioma cells.

METHODS

Expression and function of FPR in the human glioblastoma cell line U-87 were examined by reverse transcription-polymerase chain reaction (RT-PCR) and chemotaxis assays, respectively. FPR protein expression was detected in specimens from 33 human primary gliomas by immunohistochemistry. FPR short interfering (si) RNA was used to block FPR expression in U-87 cells. Cell proliferation was assessed by measuring DNA synthesis. Xenograft tumor formation and growth were measured in nude mice. Endogenous FPR agonist activity released by necrotic tumor cells was assessed by measuring FPR activation in an FPR-transfected basophil leukemia cell line and live U-87 cells. Vascular endothelial growth factor (VEGF) mRNA was assessed by RT-PCR, and VEGF protein was assessed by enzyme-linked immunosorbent assay. All statistical tests were two-sided.

RESULTS

FPR was selectively expressed by the highly malignant human glioblastoma cell line U-87 and most primary grade IV glioblastomas multiforme and grade III anaplastic astrocytomas. U-87 cells responded to the FPR agonist fMLF by chemotaxis (i.e., increased motility), increased cell proliferation, and increased production of VEGF protein. FPR siRNA substantially reduced the tumorigenicity of U-87 cells in nude mice (38 days after implantation, mean tumor volume from wild-type U-87 cells = 842 mm3, 95% confidence interval [CI] = 721 to 963 mm3; and from FPR-siRNA transfected U-87 cells = 225 mm3, 95% CI = 194 to 256 mm3; P = .001). Necrotic glioblastoma cells released a factor(s) that activated FPR in live U-87 cells.

CONCLUSIONS

FPR is expressed by highly malignant human glioma cells and appears to mediate motility, growth, and angiogenesis of human glioblastoma by interacting with host-derived agonists. Thus, FPR may represent a molecular target for the development of novel antiglioma therapeutics.

CD38 cleavage in fMLP- and IL-8-induced chemotaxis is dependent on p38 MAP kinase but independent of p44/42 MAP kinase

In this study, we examined the mechanism by which CD38 cleavage is regulated through the mitogen-activated protein (MAP) kinases after stimulation by fMLP and interleukin-8 (IL-8) in neutrophils. Both fMLP and IL-8 increased chemotaxis and decreased CD38 protein in neutrophils, but did not change CD38 mRNA levels. Both fMLP and IL-8 increased CD38 in supernatants, which was inhibitable with PMSF. fMLP stimulation resulted in phosphorylation of p38 MAP kinase and p42/44 MAP kinase (ERK). SB20358, a p38 MAP kinase inhibitor, down-regulated neutrophil chemotaxis. Conversely, PD98059, an ERK inhibitor, did not influence chemotaxis to either agonist. The addition of SB20358 blocked the decrease of CD38 on neutrophils and the increase in supernatants induced by fMLP or IL-8, whereas PD98059 did not. These findings suggest that CD38-mediated chemotaxis to fMLP or IL-8 is characterized by proteolytic cleavage of CD38 and signaling through p38 MAP kinase. Activation of the protease for cleavage appears to be a postreceptor event that is dependent on p38 MAP kinase signaling.

Innate immune traits differ between Meishan and Large White pigs

A panel of innate immune traits were compared between Meishan and Large White pigs. These pigs were of similar age and kept under the same environmental conditions to reduce non-genetically derived variation in immune traits. The animals were all apparently healthy and were not experimentally challenged with any pathogen during the study. The measures only required a small blood sample. Total white cell counts were similar between the pig breeds. However, the numbers of lymphocytes, neutrophils and monocytes differed significantly, with Meishans having higher neutrophil and monocyte counts and lower lymphocyte counts. Flow cytometric methods were used to determine quantitatively the characteristics and function of neutrophils and monocytes. Meishan neutrophils were smaller and less complex than Large White neutrophils, and phagocytosis of Escherichia coli and the ensuing oxidative burst was lower in Meishan neutrophils compared to Large White neutrophils. Monocyte phagocytosis of E. coli was significantly less than that of neutrophils in both breeds but the function of Meishan monocytes as measured by phagocytosis and tumour necrosis factor alpha (TNFalpha) release did not differ from that of Large White monocytes. Levels of acute phase proteins also differed between the breeds with a significantly higher proportion of Meishans having elevated serum amyloid A levels. However, Meishans had lower alpha(1)-acid glycoprotein levels than Large Whites and haptoglobin levels were similar. Such differences in innate immune traits may have implications in the resistance to infection by a broad range of pathogens and subsequent disease effects in these breeds. Further studies are warranted to investigate the genes underlying these traits.

Identification of functional domains in the formyl peptide receptor-like 1 for agonist-induced cell chemotaxis

Formyl peptide receptor-like 1 (FPRL1) is a seven transmembrane domain, G protein-coupled receptor that interacts with a variety of exogenous and host-derived agonists. In order to identify domains crucial for ligand recognition by FPRL1, we used chimeric receptors with segments in FPRL1 replaced by corresponding amino acid sequences derived from the prototype formyl peptide receptor FPR. The chimeric receptors were stably transfected into human embryonic kidney epithelial cells and the capacity of the cells to migrate in response to formyl peptide receptor agonists was evaluated. Our results showed that multiple domains in FPRL1 are involved in the receptor response to chemotactic agonists with the sixth transmembrane domain and the third extracellular loop playing a prominent role. Interestingly, the N-terminus and a segment between the fourth transmembrane domain and the third intracellular loop of FPRL1 are important for receptor interaction with a 42 amino acid amyloid beta peptide (Abeta42), an Alzheimer's disease-associated FPRL1 agonist, but not with MMK-1, a synthetic FPRL1 agonist, suggesting that diverse agonists may use different domains in FPRL1. Considering the potential importance of FPRL1 in inflammation and neurodegenerative diseases, the identification of functional domains in this receptor will provide valuable information for the design of specific receptor antagonists.

Human eosinophils selectively recognize and become activated by bacteria belonging to different taxonomic groups

Eosinophils are predominantly found in tissues that have an interface with the external environment and its bacterial flora, such as the gastrointestinal and respiratory tracts. Although it is not the primary function of eosinophils to phagocytose and kill bacteria, we hypothesized that they might be able to recognize and become activated by microorganisms that enter the normally sterile tissues where they reside. The aim of this study was to evaluate whether human eosinophils get universally activated by bacteria or if they discriminate between bacteria derived from different phylogenetic groups. Eleven bacterial species representative of different taxonomic groups were examined. A hierarchy was seen among the bacterial species regarding their capacity to activate eosinophils. Furthermore, several eosinophilic activation patterns were evoked by the different bacterial species. The strongest eosinophil activator, Escherichia coli, elicited chemotaxis, degranulation and respiratory burst. Low numbers of bacteria caused the release of the granule proteins major basic protein and eosinophil peroxidase, whereas high numbers were required for the release of eosinophil cationic protein (ECP). Eosinophils did not seem to discriminate between gram-positive and gram-negative bacteria, unlike monocytes. However, the release of ECP was mainly seen after stimulation with gram-negative species. Blockade of the formyl peptide receptor partially inhibited bacterial activation of eosinophils, implicating its involvement in this activity. We propose that the presence of defined bacterial species in the normally sterile tissues inhabited by eosinophils may constitute danger signals to eosinophils. This may be of importance in the perpetuation of allergic inflammation.

Regulation of lipopolysaccharide-inducible genes by MyD88 and Toll/IL-1 domain containing adaptor inducing IFN-β

Macrophages recognize lipopolysaccharide (LPS) by Toll-like receptor 4 and activate inflammatory responses by inducing expression of various genes. TLR4 activates intracellular signaling pathways via TIR domain containing adaptor molecules, MyD88, and Toll/IL-1 domain containing adaptor inducing IFN-beta (TRIF). Although macrophages lacking MyD88 or TRIF showed impaired cytokine production, activation of intracellular signaling molecules still occurred in response to LPS in these cells. In the present study, we implemented cDNA microarrays to investigate the contribution of MyD88 and TRIF in gene expression induced by LPS stimulation. Whereas wild-type macrophages induced 148 genes in response to LPS, macrophages lacking both MyD88 and TRIF did not upregulate any genes in response to LPS. Surprisingly, 80 LPS-inducible genes were redundantly regulated by either MyD88 or TRIF. In contrast, proinflammatory cytokines and chemokines were critically regulated by MyD88 or TRIF alone. Genes critically regulated by MyD88 alone tend to be induced quickly after LPS stimulation and regulated by mRNA stability as well as transcription. Genes known to be induced by type I interferons were simply dependent on TRIF for their expression. Taken together, MyD88 and TRIF play both redundant and distinct roles in LPS-induced gene expression.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

N-terminal residues of the chemotaxis inhibitory protein of Staphylococcus aureus are essential for blocking formylated peptide receptor but not C5a receptor

Staphylococcus aureus excretes a factor that specifically and simultaneously acts on the C5aR and the formylated peptide receptor (FPR). This chemotaxis inhibitory protein of S. aureus (CHIPS) blocks C5a- and fMLP-induced phagocyte activation and chemotaxis. Monoclonal anti-CHIPS Abs inhibit CHIPS activity against one receptor completely without affecting the other receptor, indicating that two distinct sites are responsible for both actions. A CHIPS-derived N-terminal 6 aa peptide is capable of mimicking the anti-FPR properties of CHIPS but has no effect on the C5aR. Synthetic peptides in which the first 6 aa are substituted individually for all other naturally occurring amino acids show that the first and third residue play an important role in blocking the FPR. Using an Escherichia coli expression system, we created mutant CHIPS proteins in which these amino acids are substituted. These mutant proteins have impaired or absent FPR- but still an intact C5aR-blocking activity, indicating that the loss of the FPR-blocking activity is not caused by any structural impairment. This identifies the first and third amino acid, both a phenylalanine, to be essential for CHIPS blocking the fMLP-induced activation of phagocytes. The unique properties of CHIPS to specifically inhibit the FPR with high affinity (kd=35.4 +/- 7.7 nM) could be an important new tool to further stimulate the fundamental research on the mechanisms underlying the FPR and its role in disease processes.

Formyl‐peptide receptor is not involved in the protection afforded by annexin 1 in murine acute myocardial infarct

Recent interest in the annexin 1 field has come from the notion that specific G-protein-coupled receptors, members of the formyl-peptide receptor (FPR) family, appear to mediate the anti-inflammatory actions of this endogenous mediator. Administration of the annexin 1 N-terminal derived peptide Ac2-26 to mice after 25 min ischemia significantly attenuated the extent of acute myocardial injury as assessed 60 min postreperfusion. Evident at the dose of 1 mg/kg (approximately 9 nmol per animal), peptide Ac2-26 cardioprotection was intact in FPR null mice. Similarly, peptide Ac2-26 inhibition of specific markers of heart injury (specifically myeloperoxidase activity, CXC chemokine KC contents, and endogenous annexin 1 protein expression) was virtually identical in heart samples collected from wild-type and FPR null mice. Mouse myocardium expressed the mRNA for FPR and the structurally related lipoxin A4 receptor, termed ALX; thus, comparable equimolar doses of two ALX agonists (W peptide and a stable lipoxin A4 analog) exerted cardioprotection in wild-type and FPR null mice to an equal extent. Curiously, marked (>95%) blood neutropenia produced by an anti-mouse neutrophil serum did not modify the extent of acute heart injury, whereas it prevented the protection afforded by peptide Ac2-26. Thus, this study sheds light on the receptor mechanism(s) mediating annexin 1-induced cardioprotection and shows a pivotal role for ALX and circulating neutrophil, whereas it excludes any functional involvement of mouse FPR. These mechanistic data can help in developing novel therapeutics for acute cardioprotection.

Pneumolysin potentiates oxidative inactivation of alpha-1-proteinase inhibitor by activated human neutrophils

This study was designed to investigate the effects of the Streptococcus pneumoniae-derived, pro-inflammatory toxin, pneumolysin (8.37 and 41.75 ng/ml), on the oxidative inactivation of alpha-1-protease inhibitor (API) by chemoattractant-activated human neutrophils in vitro. The elastase inhibitory capacity (EIC) of API in supernatants from unstimulated neutrophils, neutrophils treated with pneumolysin only, or with the chemoattractant FMLP (1 microM) only, or the combination of the toxin with FMLP was measured by a colorimetric procedure based on the activity of added porcine elastase. The EIC of API was unaffected by exposure to pneumolysin only, unstimulated neutrophils, or neutrophils treated with pneumolysin only. However, exposure to FMLP-activated neutrophils resulted in a reduction of the EIC of API, which was significantly (P<0.05) augmented by pneumolysin (mean reductions of 16%, 43% and 83% for FMLP only and in combination with 8.37 and 41.75 ng/ml pneumolysin, respectively), and was attenuated by wortmannin (1 microM), an inhibitor of NADPH oxidase, the oxidant-scavenger methionine (100 microM), and depletion of Ca2+ from the cell-suspending medium. These pro-proteolytic interactions of pneumolysin with chemoattractant-activated neutrophils may contribute to the invasiveness of the pneumococcus.

Identification of neutrophil granule protein cathepsin G as a novel chemotactic agonist for the G protein-coupled formyl peptide receptor

The antimicrobial and proinflammatory neutrophil granule protein cathepsin G (CaG) has been reported as a chemoattractant for human phagocytic leukocytes by using a putative G protein coupled receptor. In an effort to identify potential CaG receptor(s), we found that CaG-induced phagocyte migration was specifically attenuated by the bacterial chemotactic peptide fMLP, suggesting these two chemoattractants might share a receptor. In fact, CaG chemoattracts rat basophilic leukemia cells (RBL cells) expressing the high affinity human fMLP receptor FPR, but not parental RBL cells or cells transfected with other chemoattractant receptors. In addition, a specific FPR Ab and a defined FPR antagonist, cyclosporin H, abolished the chemotactic response of phagocytes and FPR-transfected cells to CaG. Furthermore, CaG down-regulated the cell surface expression of FPR in association with receptor internalization. Unlike fMLP, CaG did not induce potent Ca(2+) flux and was a relatively weaker activator of MAPKs through FPR. Yet CaG activated an atypical protein kinase C isozyme, protein kinase Czeta, which was essential for FPR to mediate the chemotactic activity of CaG. Thus, our studies identify CaG as a novel, host-derived chemotactic agonist for FPR and expand the functional scope of this receptor in inflammatory and immune responses.

The annexin 1 receptor(s): is the plot unravelling?

Recent studies have proposed a functional link between annexin 1 (ANXA1), an endogenous anti-inflammatory mediator, and receptors of the formyl-peptide family. In particular, exogenous and endogenous ANXA1 and its peptidomimetics interact with one member of this family, the formyl-peptide-receptor-like 1. Further analyses of the interactions between ANXA1 and this and other members of this receptor family, and a better characterization of the ANXA1 receptor systems in models of inflammation, might clarify their mechanism of anti-inflammatory effects. This line of research will facilitate the development of ANXA1 mimetics and take advantage of >20 years of biological research into the functions of this glucocorticoid-modulated protein.

Silencing the formylpeptide receptor FPR by short-interfering RNA

A double-stranded short-interfering RNA (siRNA) was designed to attenuate the expression and function of the formylpeptide receptor FPR, a G protein-coupled receptor mediating migration and activation of phagocytic leukocytes in response to bacterial chemotactic formylpeptides. Retrovirus-based constructs were generated to introduce FPR-siRNA into a rat leukemia cell line transfected to overexpress FPR. Cells infected with FPR-siRNAT28, which targets the nucleotides 926 to 944 of FPR mRNA corresponding to the third extracellular loop of the putative receptor protein, showed significantly reduced expression of FPR mRNA and protein, in association with impaired calcium mobilization and chemotactic responses to peptide agonists. Direct transduction of synthetic FPR-siRNAT28 into human macrophages also inhibited the expression of FPR and abrogated cell chemotaxis and the release of superoxide anions induced by the bacterial formylpeptide. FPR-siRNA additionally abrogated the expression and function of FPR in a human malignant glioma cell line. Our study demonstrates successful application of siRNA to silence a G protein-coupled chemoattractant receptor involved in inflammation and suggests the potential to use this approach in studies of receptor regulation and prevention of undesirable side effects associated with FPR activation.

Chemotaxis and calcium responses of phagocytes to formyl peptide receptor ligands is differentially regulated by cyclic ADP ribose

Cyclic ADP ribose (cADPR) is a calcium-mobilizing metabolite that regulates intracellular calcium release and extracellular calcium influx. Although the role of cADPR in modulating calcium mobilization has been extensively examined, its potential role in regulating immunologic responses is less well understood. We previously reported that cADPR, produced by the ADP-ribosyl cyclase, CD38, controls calcium influx and chemotaxis of murine neutrophils responding to fMLF, a peptide agonist for two chemoattractant receptor subtypes, formyl peptide receptor and formyl peptide receptor-like 1. In this study, we examine whether cADPR is required for chemotaxis of human monocytes and neutrophils to a diverse array of chemoattractants. We found that a cADPR antagonist and a CD38 substrate analogue inhibited the chemotaxis of human phagocytic cells to a number of formyl peptide receptor-like 1-specific ligands but had no effect on the chemotactic response of these cells to ligands selective for formyl peptide receptor. In addition, we show that the cADPR antagonist blocks the chemotaxis of human monocytes to CXCR4, CCR1, and CCR5 ligands. In all cases, we found that cADPR modulates intracellular free calcium levels in cells activated by chemokines that induce extracellular calcium influx in the apparent absence of significant intracellular calcium release. Thus, cADPR regulates calcium signaling of a discrete subset of chemoattractant receptors expressed by human leukocytes. Since many of the chemoattractant receptors regulated by cADPR bind to ligands that are associated with clinical pathology, cADPR and CD38 represent novel drug targets with potential application in chronic inflammatory and neurodegenerative disease.

N-Formyl peptide receptor subtypes in human neutrophils activate L-plastin phosphorylation through different signal transduction intermediates

We investigated the coupling of the fMLP (N -formyl-L-methionyl-L-leucyl-L-phenylalanine; 'chemotactic peptide') receptor with phosphorylation of the actin-binding protein L-plastin in neutrophils. Using two-dimensional IEF (isoelectric focusing)/PAGE and MALDI-TOF (matrix-assisted laser desorption ionization-time-of-flight)-MS, L-plastin was identified as a major phosphoprotein in fMLP-stimulated neutrophils whose phosphorylation was dependent on phosphoinositide 3-kinase, PLD (phospholipase D) and PKC (protein kinase C) activity. Two fMLP receptor subtypes were identified in neutrophils, characterized by a distinct sensitivity to fMLP and antagonistic peptides. Both receptor subtypes induced the phosphorylation of L-plastin. L-plastin phosphorylation induced by low-affinity fMLP receptors involves an action of phosphoinositide 3-kinase, PLD and PKC isotypes. In contrast, none of these intermediates are utilized by high-affinity fMLP receptors in the phosphorylation of L-plastin. However, the PKC inhibitor Ro-31-8220 inhibits L-plastin phosphorylation induced by the high-affinity fMLP receptor. Thus, an as yet unknown Ro-31-8220-sensitive kinase regulates L-plastin phosphorylation in response to the high-affinity fMLP receptor. The results suggest a model in which receptor subtypes induce a similar endpoint event through different signal-transduction intermediates. This may be relevant in the context of cell migration in which one receptor subpopulation may become desensitized in a concentration gradient of chemoattractant.

Listeria monocytogenes mutants that fail to compartmentalize listerolysin O activity are cytotoxic, avirulent, and unable to evade host extracellular defenses

Listeria monocytogenes is a facultative intracellular bacterial pathogen that escapes from a phagosome and grows in the host cell cytosol. Escape of the bacterium from the phagosome to the cytosol is mediated by the bacterial pore-forming protein listeriolysin O (LLO). LLO has multiple mechanisms that optimize activity in the phagosome and minimize activity in the host cytosol. Mutants that fail to compartmentalize LLO activity are cytotoxic and have reduced virulence. We sought to determine why cytotoxic bacteria have attenuated virulence in the mouse model of listeriosis. In this study, we constructed a series of strains with mutations in LLO and with various degrees of cytotoxicity. We found that the more cytotoxic the strain in cell culture, the less virulent it was in mice. Induction of neutropenia increased the relative virulence of the cytotoxic strains 100-fold in the spleen and 10-fold in the liver. The virulence defect was partially restored in neutropenic mice by adding gentamicin, an antibiotic that kills extracellular bacteria. Additionally, L. monocytogenes grew more slowly in extracellular fluid (mouse serum) than within tissue culture cells. We concluded that L. monocytogenes controls the cytolytic activity of LLO to maintain its nutritionally rich intracellular niche and avoid extracellular defenses of the host.

The hemopoietic Rho/Rac guanine nucleotide exchange factor Vav1 regulates N-formyl-methionyl-leucyl-phenylalanine-activated neutrophil functions

Vav1 is a hemopoietic-specific Rho/Rac guanine nucleotide exchange factor that plays a prominent role in responses to multisubunit immune recognition receptors in lymphoid cells, but its contribution to regulation of neutrophil functions is unknown. Activated Rho family GTPases are critical participants in neutrophil signaling cascades initiated by binding of FMLP and other chemoattractants to their cognate G protein-coupled receptors. Therefore, we investigated whether Vav1 regulates chemoattractant-induced responses in neutrophils. We found that superoxide production elicited by FMLP in Vav1(-/-) murine neutrophils isolated from either bone marrow or from peritoneal exudates was substantially reduced compared with that of wild type. Filamentous actin generation in FMLP-stimulated Vav1(-/-) neutrophils was also markedly reduced, whereas it was normal in response to IL-8 or leukotriene B(4). FMLP induced tyrosine phosphorylation of Vav1, whereas IL-8 or leukotriene B(4) did not, correlating with the requirement for Vav1 in chemoattractant-stimulated filamentous actin generation. Neutrophil motility in vitro and neutrophil mobilization into peripheral blood in vivo elicited by FMLP were both decreased in Vav1(-/-) mice. Hence, this study defines a new role for Vav1 in regulating granulocytic leukocytes as well as linking Vav1 to specific cellular responses downstream of a seven transmembrane domain receptor.

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

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

Subinhibitory concentrations of the deformylase inhibitor actinonin increase bacterial release of neutrophil-activating peptides: a new approach to antimicrobial chemotherapy

Bacterial protein synthesis starts with a formylated methionine residue, and this residue is sequentially cleaved away by a unique peptide deformylase (PDF) and a methionine aminopeptidase to generate mature proteins. The formylation-deformylation of proteins is a unique hallmark of bacterial metabolism and has recently become an attractive target for the development of antimicrobial agents. The innate immune system uses the formylation of bacterial proteins as a target, and professional phagocytes, e.g., neutrophils, express specific receptors for bacterium-derived formylated peptides. Activation of formyl peptide receptors (FPR) mediates neutrophil migration and the release of oxygen radicals and other antimicrobial substances from these cells. We hypothesize that the use of a PDF inhibitor would increase the production of proinflammatory peptides from the bacteria and thus trigger a more pronounced innate immune response. We tested this hypothesis by exposing Escherichia coli to subinhibitory doses of the PDF inhibitor actinonin and show that actinonin indeed increases the production and secretion of neutrophil-activating peptides that activate human neutrophils through FPR. These findings could be potentially used as a new approach to antibacterial chemotherapy.

Expression and molecular pharmacology of the mouse CRTH2 receptor

Prostaglandin D2 (PGD2), the predominant prostanoid produced by activated mast cells, is implicated in a variety of allergic diseases. PGD2 exerts its effects through two G-protein coupled receptors, DP and CRTH2. PGD2 mediates chemotaxis of eosinophils, basophils, and Th2 cells via CRTH2-evoked signaling, suggesting a role for this receptor in allergic disease. To characterize the mouse CRTH2 ortholog (mCRTH2), we amplified the mCRTH2 receptor gene and expressed it in HEK293 cells. Saturation ligand binding isotherms demonstrated high-affinity binding of [3H]PGD2, with a Kd of 8.8 +/- 0.8 nM. Competition binding assays with a panel unlabeled prostanoids demonstrated an order of affinity of 13,14-dihydro-15-keto-PGD2 (DK-PGD2) >or= 15-deoxy-Delta12,14-PGJ2 (15d-PGJ2) >or= PGD2 >or= PGJ2. [3H]PGD2 binding was also displaced by the nonsteroidal anti-inflammatory drug indomethacin, with a Ki value of 1.04 +/- 0.13 microM. No [3H]PGD2 displacement was detected using fluribrofen, ibuprofen, or aspirin as competitors at concentrations of up to 30 microM. PGD2, DK-PGD2, 15d-PGJ2, and indomethacin each inhibited intracellular cAMP generation in stable transfectant ER293/mCRTH2 cells through a pertussis toxin (PTX) sensitive pathway, consistent with mCRTH2 coupling to a Gi heterotrimeric G-protein. Activation of mCRTH2 elicited chemotaxis of ER293/mCRTH2 cells in response to PGD2, indomethacin, and 15d-PGJ2. mCRTH2-dependent chemotaxis was inhibited by PTX and wortmannin, indicating dependence on Gi and PI 3-kinase signal transduction pathways. These data provide the first pharmacological and functional characterization of the mouse CRTH2 receptor.

Leukocyte antiadhesive actions of annexin 1: ALXR- and FPR-related anti-inflammatory mechanisms

Recent investigations conducted with human neutrophils have indicated an involvement for the receptor for formylated peptides, termed FPR, and its analog FPRL1 (or ALXR because it is the receptor for the endogenous ligand lipoxin A(4)) in the in vitro inhibitory actions of the glucocorticoid-regulated protein annexin 1 and its peptidomimetics. To translate these findings in in vivo settings, we have used an ischemia/reperfusion (I/R) procedure to promote leukocyte-endothelium interactions in the mouse mesenteric microcirculation. In naive mice, the annexin 1 mimetic peptide Ac2-26 (20 to 100 microg administered intravenously prior to reperfusion) abolished I/R-induced cell adhesion and emigration, but not cell rolling. In FPR-deficient mice, peptide Ac2-26 retained significant inhibitory actions (about 50% of the effects in naive mice), and these were blocked by an FPR antagonist, termed butyloxycarbonyl-Phe-Leu-Phe-Leu-Phe, or Boc2. In vitro, neutrophils taken from these animals could be activated at high concentrations of formyl-Met-Leu-Phe (30 microM; fMLP), and this effect was blocked by cell incubation with peptide Ac2-26 (66 microM) or Boc2 (100 microM). FPR-deficient neutrophils expressed ALXR mRNA and protein. Both ALXR agonists, lipoxin A(4) and peptide Ac2-26, provoked detachment of adherent leukocytes in naive as well as in FPR-deficient mice, whereas the CXC chemokine KC or fMLP were inactive. The present findings demonstrate that endogenous regulatory autocoids such as lipoxin A(4) and annexin 1-derived peptides function to disengage adherent cells during cell-cell interactions.

Functional differences between human formyl peptide receptor isoforms 26, 98, and G6

The formyl peptide receptor (FPR) is expressed in neutrophils, couples to G(i)-proteins and activates phospholipase C, chemotaxis and cytotoxic cell functions. FPR isoforms 26, 98, and G6 differ from each other in amino acids 101, 192 and 346 (FPR-26: V101, N192, E346; FPR-98: L101, N192, A346; FPR-G6: V101, K192, A346), but the functional significance of those structural differences is unknown. In order to address this question, we analyzed FPR-26, FPR-98 and FPR-G6 by co-expressing recombinant FLAG epitope-tagged FPRs with the G-protein G(i)alpha(2)beta(1)gamma(2) in Sf9 insect cells and measured high-affinity agonist binding and guanosine 5'- O-(3-thiotriphosphate) (GTPgammaS) binding. The B(max) values of high-affinity agonist binding with FPR-98 and FPR-G6 were much lower than with FPR-26. FPR-98 and FPR-G6 activated considerably fewer G(i)-proteins, and were much less constitutively active, than FPR-26. Whereas FPR-26 migrated as a monomer in SDS polyacrylamide electrophoresis, FPR-98 and FPR-G6 migrated as dimers and tetramers. In terms of immunoreactivity, FRP-98 and FPR-G6 were expressed at higher levels than FPR-26. Single amino acid exchanges at positions 101 (V-->L), 192 (N-->K) and 346 (E-->A) in FPR-26 revealed that E346 accounts for FPR-26 migrating as a monomer and the high constitutive activity of FPR-26. The V101L, N192K and E346A exchanges all reduced high-affinity agonist binding and the number of G(i)-proteins activated by FPR-26. We conclude that (i) FPR isoforms 98 and G6 exhibit a partial G(i)-protein coupling defect relative to FPR-26 and that (ii) E346 critically determines constitutive activity, G(i)-protein coupling and physical state of FPR-26.

Mutations of F110 and C126 of the formyl peptide receptor interfere with G-protein coupling and chemotaxis

BACKGROUND

Localized aggressive periodontitis (LAgP) is a disease characterized by rapid loss of alveolar bone in teeth of otherwise healthy patients. Neutrophils from LAgP patients have been shown to exhibit diminished chemotaxis and low levels of formyl peptide receptor (FPR) surface expression. A recent study has associated LAgP with 2 polymorphisms in the FPR: 110Phe-->Ser and 126Cys-->Trp.

METHODS

We transfected Chinese hamster ovary cells with wtFPR, FPR-110Phe-->Ser, FPR-126Cys-->Trp, or FPR-110Phe-->Ala and determined their surface expression of FPR, their ligand binding affinity, their G-protein coupling, and their chemotaxis toward N-formyl-methionyl-leucyl-phenylalanine (FMLP).

RESULTS

FPR-110Phe-->Ser mutants failed to show any significant surface expression or chemotaxis toward FMLP. FPR-126Cys-->Trp mutants exhibited slightly lower than normal binding affinity, markedly lower G-protein coupling response, and markedly lower chemotaxis toward FMLP than that observed with wtFPR. We also analyzed another FPR-Phe110 mutant, FPR-110Phe-->Ala, to ascertain what the effect of mutating this residue might be in a mutant that could be expressed on the cell surface. The FPR-110Phe-->Ala mutant demonstrated markedly lower surface expression, normal ligand binding affinity, markedly lower G-protein coupling, and markedly lower chemotaxis toward FMLP.

CONCLUSIONS

Our data substantiate the hypothesis that the chemotactic defects observed in LAgP patients are due at least in part to molecular alterations in the FPR. The FPR-110Phe-->Ser polymorphism appears to be more defective than the FPR-126Cys-->Trp polymorphism, indicating that patients with the former polymorphism might be expected to exhibit a more severe form of aggressive periodontitis.

Induction of HIV-1 long terminal repeat-mediated transcription by Neisseria gonorrhoeae

Gonorrhoea enhances the transmission of HIV through increased viral shedding and the increased probability of seroconversion among previously HIV-negative individuals. However, the mechanism(s) underlying these influences remain poorly understood. We demonstrated that exposure to Neisseria gonorrhoeae induces the nuclear factor kappa B-dependent transcription from the HIV-1 long terminal repeat in derivatives of the Jurkat CD4 T cell line. These data suggest that gonococcal infection directly impacts HIV-1 transmission through the localized stimulation of viral expression.

The formyl peptide N-formyl-methionyl-leucyl-phenylalanine downregulates the expression of FcgammaRs in interferon-gamma-activated monocytes/macrophages in vitro and in vivo

N-Formyl peptides are cleavage products of bacterial and mitochondrial proteins that have pro-inflammatory activities and play an important role in antibacterial host defence. FcgammaRI is a receptor for the Fc portion of immunoglobulin G expressed in monocytes that mediates cytotoxicity and is upregulated by interferon-gamma (IFN-gamma) and interleukin-10 (IL-10). In this report, we demonstrate that N-formyl-methionyl-leucyl-phenylalanine (FMLP) downregulates the expression of FcgammaRI in IFN-gamma-treated monocytes, but not in IL-10-treated monocytes. We determine that supernatants obtained from monocytes treated with IFN-gamma and then exposed to FMLP induce the downregulation of FcgammaRI in naïve monocytes. This effect is abrogated by the protease inhibitors phenylmethylsulphonyl fluoride and phosphoramidon, which inhibit serine and metalloproteases, respectively. Supernatants from FMLP-treated neutrophils also induce the downregulation of FcgammaRI, when added to naïve monocytes. Similar observations were obtained in vivo in a mouse model of chronic inflammation. In vivo, FMLP also downregulates the expression of FcgammaRs in IFN-gamma-activated macrophages. Our results support the existence of a new mechanism through which FMLP could modulate the activity of monocytes/macrophages during bacterial infections.

Innate immunity is regulated by CD38, an ecto-enzyme with ADP-ribosyl cyclase activity

Through its production of cyclic adenosine diphosphate ribose, the ecto-enzyme CD38 regulates calcium mobilization in neutrophils responding to some, but not all, chemoattractants. This signaling defect results in reduced chemotaxis of CD38-deficient neutrophils to bacterial peptides and increased susceptibility of CD38-deficient mice to bacterial infections.

Effects of Shuanghuanglian and Qingkailing, two multi-components of traditional Chinese medicinal preparations, on human leukocyte function

Qingkailing (QKL) and Shuanghuanglian (SHHL) are two commonly used Chinese herbal preparations with reported antiinflammatory activity. The effects of these two preparations on the capacity of staphylococcal toxic shock syndrome toxin 1 (TSST-1) to stimulate the production of cytokines (IL-1beta, IL-6, TNF-alpha, IFN-gamma) and chemokines (MIP-1alpha, MIP-1beta and MCP-1) by peripheral blood mononuclear cell (PBMC) was tested. We also evaluated their effect on LPS-stimulated NF-kappaB transcriptional activity in a THP-1 cell line, and on human monocyte chemotactic response to chemoattractants. Non-cytotoxic concentrations of QKL (0.1 to approximately 2%) and SHHL (6 to approximately 120 microg) significantly inhibited production of cytokines and chemokines in a dose-dependent manner (P < 0.05). Both, QKL at 1:100 and SHHL at 60 microg/ml, markedly inhibited RANTES, MIP-1alpha, SDF-1alpha and fMLP induced human monocyte migration (P < 0.05 or 0.01). QKL (1%) did not inhibit monocyte chemotaxis induced by super-or sub-optimal concentrations of fMLP (10(-5), 10(-6) and 10(-10) M), but only inhibited chemotaxis induced by optimal concentrations of fMLP at 10(-7), 10(-8) and 10(-9) M. QKL (0.1% or 1%) and SHHL (6 or 60 microg/ml) markedly inhibited LPS-induced NF-kappaB activity in THP-1 cells. The results suggested that the pharmacological basis for the antiinflammatory effects of QKL and SHHL is the result of suppression of NF-kappaB regulated gene transcription, leading to suppressed production of proinflammatory cytokine and chemokine. Interference with leukocyte chemotaxis also contributes to the antiinflammatory and immunomodulating effects of these medicinals. Identification of the responsible components in these two herbal preparations may yield compounds suitable for structural modification into potent novel drugs.

Up-Regulation of FPR2, a Chemotactic Receptor for Amyloid β 1–42 (Aβ42), in Murine Microglial Cells by TNFα

Human FPRL1 and its mouse homologue FPR2 are functional receptors for several exogenous and host-derived chemotactic peptides, including amyloid beta(42) (A beta(42)), a critical pathogenic factor in Alzheimer's disease. We investigated the effect of TNF alpha on the expression and function of FPR2 in mouse microglial cells, a crucial inflammatory cell type in the CNS. Primary murine microglia and a cell line N9 in resting state expressed low levels of FPR2 gene and lacked the response to chemotactic agonists for this receptor. Incubation with TNF alpha, however, increased microglial expression of FPR2 gene, in association with potent chemotactic responses to FPR2-specific agonists including A beta(42). The effect of TNF alpha was dependent on the p55 TNF alpha receptor and activation of MAP kinase p38. TNF alpha concomitantly down-regulated microglial response to the chemokine SDF-1 alpha. Thus, by selectively up-regulating FPR2 in microglia, TNF alpha has the capacity to amplify host response in inflammatory diseases in the CNS.

The N-formylpeptide receptor (FPR) and a second G(i)-coupled receptor mediate fMet-Leu-Phe-stimulated activation of NADPH oxidase in murine neutrophils

N-Formylypeptides such as fMet-Leu-Phe (fMLF) potently induce superoxide production through NADPH oxidase activation. The receptors that mediate this response have not been defined. Here, we provide definitive proof using a mouse model that formyl peptide receptor (FPR) is a receptor, but not the only receptor, that mediates fMLF-induced oxidase activation. In wild-type (FPR(+/+)) mouse neutrophils, superoxide production is dependent on the concentration of fMLF with an EC(50) of approximately 5 microM and a peak at approximately 50 microM. In contrast, FPR-deficient (FPR(-/-)) mouse neutrophils produced markedly less superoxide with an EC(50) of approximately 50 microM and a peak at approximately 200 microM. Yet, FPR(+/+) and FPR(-/-) neutrophils showed similar oxidase activation kinetics and G(i) protein-dependent pharmacological sensitivities. These results suggested that a second receptor, likely FPR2, mediates superoxide production at high concentrations of fMLF. This less sensitive second pathway may permit continued oxidant generation in response to formyl peptides when FPR is desensitized in high concentrations of the chemotactic gradient.

C- and N-terminal residue effect on peptide derivatives' antagonism toward the formyl-peptide receptor

The biological action of several X-Phe-D-Leu-Phe-D-Leu-Z (X=3',5'-dimethylphenyl-ureido; Z=Phe, Lys, Glu, Tyr) analogues was analysed on human neutrophils to evaluate their ability to antagonize formyl-peptide receptors. X-Phe-D-Leu-Phe-D-Leu-Phe analogues obtained as C-terminal olo or amido derivatives and T-Phe-D-Leu-Phe-D-Leu-Phe analogues (T=thiazolyl-ureido) were also analysed. The activities of pentapeptide derivatives were compared with those of X-Phe-D-Leu-Phe-D-Leu-Phe chosen as reference antagonist. Our results demonstrate that X-Phe-D-Leu-Phe-D-Leu-Phe-olo, X-Phe-D-Leu-Phe-D-Leu-Glu and X-Phe-D-Leu-Phe-D-Leu-Tyr are more active antagonists than X-Phe-D-Leu-Phe-D-Leu-Phe. The presence of Lys (X-Phe-D-Leu-Phe-D-Leu-Lys) seems, instead, to inhibit the formyl-peptide receptor antagonist properties. The presence of the N-terminal thiazolyl-ureido group seems to considerably contribute to the receptor antagonist properties of T-Phe-D-Leu-Phe-D-Leu-Phe-OH. The introduction of the C-terminal methyl ester (T-Phe-D-Leu-Phe-D-Leu-Phe-OMe) or amido group (X-Phe-D-Leu-Phe-D-Leu-Phe-NH2) appears detrimental for the affinity and formyl-peptide receptor antagonist properties of the Phe-D-Leu-Phe-D-Leu-Phe derivatives. The examined peptides inhibit superoxide anion production and lysozyme release more efficaciously than neutrophil chemotaxis.

Receptors for chemotactic formyl peptides as pharmacological targets

Leukocytes accumulate at sites of inflammation and immunological reaction in response to locally existing chemotactic mediators. N-formyl peptides, such as fMet-Leu-Phe (fMLF), are some of the first identified and most potent chemoattractants for phagocytic leukocytes. In addition to the bacterial peptide fMLF and the putative endogenously produced formylated peptides, a number of novel peptide agonists have recently been identified that selectively activate the high-affinity fMLF receptor FPR and/or its low-affinity variant FPRL1, both of which belong to the seven-transmembrane (STM), G protein-coupled receptor (GPCR) superfamily. These agonists include peptide domains derived from the envelope proteins of human immunodeficiency virus type 1 (HIV-1) and at least three amyloidogenic polypeptides, the human acute phase protein serum amyloid A, the 42 amino acid form of beta amyloid peptide and a 21 amino acid fragment of human prion. Furthermore, a cleavage fragment of neutrophil granule-derived bactericidal cathelicidin, LL-37, is also a chemotactic agonist for FPRL1. Activation of formyl peptide receptors results in increased cell migration, phagocytosis, release of proinflammatory mediators, and the signaling cascade culminates in heterologous desensitization of other STM receptors including chemokine receptors CCR5 and CXCR4, two coreceptors for HIV-1. Thus, by interacting with a variety of exogenous and host-derived agonists, formyl peptide receptors may play important roles in proinflammatory and immunological diseases and constitute a novel group of pharmacological targets.

Bacterial lipopolysaccharide selectively up-regulates the function of the chemotactic peptide receptor formyl peptide receptor 2 in murine microglial cells

Receptors for the bacterial chemotactic peptide fMLP are implicated in inflammation and host defense against microbial infection. We investigated the expression and function of fMLPR in microglial cells, which share characteristics of mononuclear phagocytes and play an important role in proinflammatory responses in the CNS. The expression of the genes encoding formyl peptide receptor (FPR)1 and FPR2, the high- and low-affinity fMLPR, was detected in a murine microglial cell line N9, but these cells did not respond to chemotactic agonists known for these receptors. N9 cells incubated with bacterial LPS increased the expression of fMLPR genes and developed a species of specific, but low-affinity, binding sites for fMLP, in association with marked calcium mobilization and chemotaxis responses to fMLP in a concentration range that typically activated the low-affinity receptor FPR2. In addition, LPS-treated N9 cells were chemoattracted by two FPR2-specific agonists, the HIV-1 envelope-derived V3 peptide, and the 42 aa form of the amyloid beta peptide which is a pathogenic agent in Alzheimer's disease. Primary murine microglial cells also expressed FPR1 and FPR2 genes, but similar to N9 cells, exhibited FPR2-mediated activation only after LPS treatment. In contrast to its effect on the function of FPR2, LPS reduced N9 cell binding and biological responses to the chemokine stromal cell-derived factor-1alpha. Thus, LPS selectively modulates the function of chemoattractant receptors in microglia and may promote host response in inflammatory diseases in the CNS.

The endogenous opioid spinorphin blocks fMet-Leu-Phe-induced neutrophil chemotaxis by acting as a specific antagonist at the N-formylpeptide receptor subtype FPR

Spinorphin is an endogenous heptapeptide (leucylvalylvalyltyrosylprolyltryptophylthreonine), first isolated from bovine spinal cord, whose sequence matches a conserved region of beta-hemoglobin. Also referred to as LVV-hemorphin-4 and a member of the nonclassical opioid hemorphin family, spinorphin inhibits enkephalin-degrading enzymes and is analgesic. Recently, spinorphin was reported to block neutrophil activation induced by the chemotactic N-formylpeptide N-formylmethionylleucylphenylalanine (fMLF), suggesting a potential role as an endogenous negative regulator of inflammation. Here we use both gain- and loss-of-function genetic tests to identify the specific mechanism of spinorphin action on neutrophils. Spinorphin induced calcium flux in normal mouse neutrophils, but was inactive in neutrophils from mice genetically deficient in the fMLF receptor subtype FPR (N-formylpeptide receptor). Consistent with this, spinorphin induced calcium flux in human embryonic kidney 293 cells transfected with mouse FPR, but had no effect on cells expressing the closely related fMLF receptor subtype FPR2. Despite acting as a calcium-mobilizing agonist at FPR, spinorphin was a weak chemotactic agonist and effectively blocked neutrophil chemotaxis induced by fMLF at concentrations selective for FPR. Spinorphin did not affect mouse neutrophil chemotaxis induced by concentrations of fMLF that selectively activate FPR2. Thus, spinorphin blocks fMLF-induced neutrophil chemotaxis by acting as a specific antagonist at the fMLF receptor subtype FPR.

PUMA-G, an IFN-gamma-inducible gene in macrophages is a novel member of the seven transmembrane spanning receptor superfamily

IFN-gamma is a key immunoregulatory cytokine that plays a predominant role in innate immunity. By employing PCR-Select to search for genes differentially expressed in IFN-gamma/TNF-alpha stimulated macrophages, we identified a novel IFN-gamma-induced transcript designated PUMA-G (protein up-regulated in macrophages by IFN-gamma). PUMA-G codes for a protein with seven transmembrane helices, a feature commonly shared with the G protein-coupled receptor superfamily (GPCR). The PUMA-G protein is most similar to the human orphan GPCR HM74 (73 % identity) and GPR31 (30 % identity). PUMA-G mRNA was readily induced in macrophages after stimulation with IFN-gamma, LPS, polyIC and CpG oligonucleotides. In vivo PUMA-G was up-regulated in mice suffering from microbial sepsis or from Listeria monocytogenes infection. Characterization of the genomic locus revealed an intronless PUMA-G open reading frame. Genomic Southern blot analysis indicates that PUMA-G is a single-copy gene. PUMA-G maps to mouse chromosome 5F. Confocal microscopy of transiently transfected 264.7 RAW macrophages and 293T cells with a PUMA-G-EGFP fusion construct showed predominant fluorescence at the cell surface, suggesting a localization at the cell membrane. Taken together, our data indicate that PUMA-G is a new inducible representative of GPCR, with potential importance in macrophage functions.