Isolation of a cDNA that encodes a novel granulocyte N-formyl peptide receptor.

A cDNA of 1650 base pairs was isolated by screening an HL-60 granulocyte library with an N-formyl peptide receptor (NFPR) cDNA probe under low stringency conditions. The cDNA encodes a protein of 351 amino acids tentatively named FPR2, with a calculated molecular weight of 39 kDa. Sequence analysis revealed that FPR2 is 69% identical in sequence to the human NFPR and shares extensive homology to several other chemoattractant receptors. FPR2 expressed in transfected cells mediated formyl peptide-stimulated calcium mobilization at micromolar concentrations of ligand. FPR2 messenger is detected in granulocytic HL-60 cells, but not in undifferentiated HL-60 cells. These findings suggest that FPR2 is a novel receptor for formyl peptide ligand and a new member of the chemoattractant receptor gene family.     

T21/DP107, A synthetic leucine zipper-like domain of the HIV-1 envelope gp41, attracts and activates human phagocytes by using G-protein-coupled formyl peptide receptors

A leucine zipper-like domain, T21/DP107, located in the amino terminus of the ectodomain of gp41, is crucial to the formation of fusogenic configuration of the HIV-1 envelope protein gp41. We report that the synthetic T21/DP107 segment is a potent stimulant of migration and calcium mobilization in human monocytes and neutrophils. The activity of T21/DP107 on phagocytes was pertussis toxin-sensitive, suggesting this peptide uses Gi-coupled seven-transmembrane receptor(s). Since the bacterial chemotactic peptide fMLP partially desensitized the calcium-mobilizing activity of T21/DP107 in phagocytes, we postulated that T21/DP107 might preferentially use a lower affinity fMLP receptor. By using cells transfected to express cloned prototype chemotactic N-formyl peptide receptor (FPR) or its variant, FPR-like 1 (FPRL1), we demonstrate that T21/DP107 activates both receptors but has a much higher efficacy for FPRL1. In addition, T21/DP107 at nM concentrations induced migration of FPRL1-transfected human embryonic kidney 293 cells. In contrast, fMLP did not induce significant chemotaxis of the same cells at a concentration as high as 50 microM. Although a lipid metabolite, lipoxin A4, was a high-affinity ligand for FPRL1, it was not reported to induce Ca2+ mobilization or chemotaxis in FPRL1-transfected cells. Therefore, T21/DP107 is a first chemotactic peptide agonist identified thus far for FPRL1. Our results suggest that this peptide domain of the HIV-1 gp41 may have the potential to activate host innate immune response by interacting with FPR and FPRL1 on phagocytes.     

Preparation and properties of an improved photoaffinity ligand for the N-formyl peptide receptor

A new superior photoaffinity ligand for the N-formyl peptide receptor was prepared by derivatization of N-formyl-Met-Leu-Phe-Lys with a commercially available heterobifunctional crosslinking agent. The product, N-formyl-Met-Leu-Phe-N epsilon-(2-(p-azidosalicylamido)ethyl-1,3'- dithiopropionyl)-Lys was readily synthesized and radiolabelled, and had increased specificity and stability as compared to previously used photoaffinity ligands. The ligand rapidly associated with the receptor with high affinity (Kd = 0.28 nM). Once bound, it was virtually non-dissociable (in the absence of photolysis) in an experimental time-frame (t1/2 (off) = 154 min). The covalent incorporation of the photoaffinity ligand into the receptor upon irradiation was complete within 5 min, minimizing cell damage, and the efficiency of covalent incorporation was approx. 40%. The derivative had enhanced biological activity, having an ED50 for superoxide anion production of 0.23 nM, 27-fold lower than its parent peptide. This derivative of the N-formyl peptide was useful for up to 3 months after radiolabelling, showing a progressive decline in specific activity during storage in the dark at 4 degrees C. The enhanced stability, reproducibility and solubility of the photoaffinity ligand is expected to aid in the purification of the N-formyl peptide receptor and will prove a useful tool for analysing receptor-mediated processes.     

Effects of ammonia on human neutrophil N-formyl chemotactic peptide receptor-ligand interaction and cytoskeletal association

Ammonia is a bacterial metabolite which is commonly used to alter cytoplasmic and lysosomal pH of eukaryotic cells. Here we examine its effect on external N-formyl peptide receptors of human neutrophils. Ammonia does not affect the number of N-formyl peptide receptors on the cell surface, nor the association of the ligand-receptor complex with the cytoskeleton. However, ammonia causes a marked decrease in the affinity of the chemotactic peptide receptor for its ligand. The Kd of untreated cell for the chemotactic peptide was 0.65 +/- 0.06 nM, whereas that of ammonia treated cells was 1.02 +/- 0.10 nM (Mean +/- SEM, N = 6). These results suggest that ammonia can affect external as well as internal cellular components. Since ammonia is used to alter lysosomal and cytoplasmic pH, and is a metabolite of common bacterial pathogens, these results bear directly on its use in cell biology and on its potential as a virulence factor.     

Functional expression of the bradykinin-B2 receptor cDNA in Chinese hamster lung CCL39 fibroblasts.

The bradykinin (BK) B2 receptor cDNA was synthesized by rt-PCR and transfected into the Chinese hamster lung fibroblasts, CCL39. The CCL39 do not contain the mRNA for this receptor and do not bind BK. Clones of transfected cells were screened for BK receptor mRNA, binding of BK, and for [Ca2+]i response to BK. The clones showed various levels of receptor mRNA. Scatchard analysis of three clones, B6, B5 and B1, each gave a Kd of approximately 1.0nM while the Bmax for each clone differed at 320, 38.7, and 5.39 fmoles per 10(6) cells respectively. The [Ca2+]i response of the three clones to BK decreased with the receptor number/cell. Thus, levels of mRNA, BK binding and [Ca2+]i response proved proportionally related in the transfected clones. The actions of BK and alpha-thrombin, which has an endogenous receptor in these cells, were assessed in clone B6. BK proved active but also distinct from thrombin. BK at 10nM and thrombin at 2units/ml both effectively increased cytosolic [Ca2+]i. BK at 10nM stimulated PGE2 production three fold over basal, while thrombin only marginally elevated PGE2 levels. Alone, BK stimulated a small increase in 3H-thymidine incorporation into DNA. However, in combination with insulin, BK stimulated DNA synthesis to 76% of thrombin, a potent mitogen in these cells. These results illustrate that the BK-B2 receptor cDNA can be stably transfected into a mammalian cell and can activate transmembrane signalling pathways.     

Absence of G(i) proteins in the Sf9 insect cell. Characterization of the uncoupled recombinant N-formyl peptide receptor.

We investigated the interaction of the N-formyl peptide receptor (NFPR) with G proteins in infected Sf9 insect cells expressing the recombinant NFPR. Recombinant receptor expression of up to 27 pmol/mg protein was achieved in these cells. The receptor was recognized by an antiserum raised against an NFPR carboxyl-terminal peptide, and displayed specific and saturable binding of the formyl peptide ligand fMet-Leu-[3H]Phe. Scatchard analysis of the binding data yielded a dissociation constant of approximately 62 nM, a binding affinity of 60- to 120-fold lower than that of the high affinity sites in neutrophils and in transfected mammalian cell lines expressing the NFPR. That this low binding affinity was due to a lack of receptor coupling to G protein was suggested by the failure of guanine nucleotides to regulate receptor affinity and by the lack of formyl peptide-stimulated GTPase activity in these cells. Furthermore, immunoblotting with an anti-G(i) antibody and ADP-ribosylation experiments indicated that the approximately 40-kDa G(i) alpha subunit, which couples to the NFPR in neutrophils, is not present in Sf9 cell membranes. Thus, the current study provides for the first time evidence that a major G protein is absent in the Sf9 insect cells. Potential applications of the Sf9 system for in vitro reconstitution of the NFPR-G protein interaction are discussed.     

HER2 cytoplasmic domain generates normal mitogenic and transforming signals in a chimeric receptor.

We have investigated the biological function of an unidentified human growth factor, the ligand of the putative HER2 receptor, by characterizing the signalling properties of its receptor. HER2 (or c-erbB-2), the human homolog of the rat neu proto-oncogene, encodes a transmembrane glycoprotein of the tyrosine kinase family that appears to play an important role in human breast carcinoma. Since a potential ligand for HER2 has not yet been identified, it has been difficult to analyze the biochemical properties and biological function of this cell surface protein. For this reason, we replaced the HER2 extracellular domain with the closely related ligand binding domain sequences of the epidermal growth factor (EGF) receptor, and examined the ligand-induced biological signalling potential of this chimeric HER1-2 protein. This HER1-2 receptor is targetted to the cell surface of transfected NIH 3T3 cells, forms high and low affinity binding sites, and generates normal mitogenic and cell transforming signals upon interaction with EGF or TGF alpha. The constitutive activation of wild-type HER2 in transfected NIH 3T3 cells suggests the possibility that these cells synthesize the as yet unidentified HER2 ligand and activate HER2 by an autocrine mechanism.     

Low-affinity receptor-mediated induction of superoxide by N-formyl-methionyl-leucyl-phenylalanine and WKYMVm in IMR90 human fibroblasts

We investigated in IMR90 cells the effects of N-formyl-Met-Leu-Phe (N-fMLP) and WKYMVm (W peptide) on activation of the NADPH oxidase-like enzyme. In serum-deprived human fibroblasts, exposure to 100 microM N-fMLP or 10 microM peptide W for 1 min induced both p47phox translocation and NADPH-dependent superoxide generation. These effects were in large part mediated by prevention of the rapid activation of extracellular signal-regulated kinases (ERKs) by preincubation with the MEK1 inhibitor PD098059. Furthermore, responses to N-fMLP or W peptide were inhibited by pertussis toxin, suggesting the involvement of a seven-transmembrane G protein-coupled receptor(s) for peptides. RT-PCR experiments demonstrated the expression in these cells of the low-affinity receptor FPRL1, but not the high-affinity receptor FPR. Incubation with radiolabeled WKYMVm, which had a higher efficiency on FPRL1, revealed that human fibroblasts express binding sites for 125I-WKYMVm that are specifically displaced by increasing concentrations of unlabeled ligand. Analysis of the binding data predicted a Kd of 155.99 nM and a receptor density of about 16,200 molecules/cell. HEK293 cells, which express a NADPH oxidase-like enzyme but not formyl peptide receptors, transiently transfected with FPRL1 cDNA produced superoxide on stimulation with N-fMLP or W peptide, demonstrating that this receptor is biologically functional.     

The synthetic peptide Trp-Lys-Tyr-Met-Val-D-Met is a potent chemotactic agonist for mouse formyl peptide receptor

Formyl peptides are potent neutrophil chemoattractants. In humans and rabbits, the formyl peptide receptor (FPR) binds N-formyl-Met-Leu-Phe (fMLF) with high affinity (K(d) approximately 1 nM). The mouse FPR (mFPR) is a low-affinity receptor for fMLF (K(d) approximately 100 nM); therefore, other agonists for this receptor may exist. Using mFPR-transfected rat basophilic leukemia cells, we found that a recently identified synthetic peptide Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm) is a potent agonist for mFPR. WKYMVm induced calcium mobilization with an EC(50) of 1.2-1.5 nM. Optimal chemotaxis was achieved with 1 nM of WKYMVm, but it required 100 nM of fMLF. WKYMVm stimulated rapid and potent phosphorylation of the mitogen-activated protein kinases extracellular signal-related kinases 1 and 2 when used at 50 nM. Pertussis toxin only partially blocked calcium mobilization and production of inositol 1,4,5-trisphosphate in the stimulated mFPR cells, suggesting the possibility that this receptor couples to Galpha proteins other than Gi and Go. Competitive binding and desensitization data suggest that both peptides interact with the same receptor but may use nonoverlapping binding sites because WKYMVm was unable to effectively displace [(3)H]fMLF bound to mFPR. These results provide evidence for the presence of an alternative potent agonist for mFPR, and suggest a potential usage of WKYMVm for probing the ligand-receptor interactions with the murine formyl peptide receptor homologs.     

Kinetics of N-formyl peptide receptor up-regulation during stimulation in human neutrophils

The kinetics of receptor up-regulation was examined in isolated neutrophils and in whole blood by flow cytometry during cell activation. Stimulation of neutrophils prepared without exposure to LPS with chemoattractants induced fast up-regulation of N-formyl peptide receptors and C receptor type 3 (CR3). Biphasic N-formyl peptide binding curves were detected for saturating concentrations of N-formyl peptide at 37 degrees C. The bulk of the rapid binding during the first 30 to 60 s is attributed to already expressed binding sites whereas the slow binding over the next 3 to 4 min represents a time course of receptor up-regulation. Support for this interpretation comes from conditions under which the number of binding sites and the progress of the binding curves were affected. Cells treated with LPS, which caused expression of internal N-formyl peptide receptors, exhibited rapid, monophasic binding curves with increased total binding. In LPS-untreated, calcium-depleted cells, N-formyl peptide receptor up-regulation was inhibited and rapid, monophasic binding to a smaller total number of expressed sites was observed. Cytochalasin B enhanced the total number of available N-formyl peptide receptors in LPS-untreated but not LPS-treated cells. In both cases binding was rapid and monophasic suggesting that receptors were either fully or rapidly up-regulated. Although not studied in real-time, C receptor type 3 up-regulation was similar to N-formyl peptide receptor up-regulation in response to LPS, or stimulation by N-formyl peptide, C product C5a, leukotriene B4, and platelet-activating factor in isolated cells and in whole blood. After stimulation with formyl peptide, LPS, or C product 5a, the release of vitamin B12-binding protein paralleled up-regulation of receptors. These data indicate that untreated cells up-regulate N-formyl peptide receptors during cell response at a rate of approximately 10,000/min in a calcium-dependent manner whereas LPS-treated cells already express the bulk of their receptors. In cytochalasin B-treated, degranulating cells 30,000 to 50,000 receptors were up-regulated within a minute.     

Proteinase-activated receptors (PARs)--the PAR3 Neo-N-terminal peptide TFRGAP interacts with PAR1

Thrombin activates proteinase-activated receptor (PAR)1, PAR3 and PAR4 by a unique mechanism that involves cleavage of the receptor and exposure of a new N-terminal domain acting as a tethered ligand. Synthetic peptides based on the proteolytically revealed receptor sequence can selectively activate PAR1 or PAR4 independently of receptor cleavage. However, corresponding peptides for PAR3 have not been identified thus far. Here, we demonstrate that the synthetic peptide TFRGAP representing the 1st six residues of the new amino terminus of PAR3 induced ERK activation in human A-498 carcinoma cells endogeneously expressing PAR1 and PAR3. This effect was completely abolished by single alanine substitution at positions 3, 4 and 6 in the peptide. Since the specific PAR1 antagonist RWJ 56110 completely abolished TFRGAP-induced ERK activation in A-498 cells we speculate that TFRGAP does signal MAPK via interaction with PAR1. This was underlined by experiments on PAR1-/- mouse lung fibroblasts (KOLF cells) that stably overexpress human PAR1 and PAR3, respectively. While TFRGAP was without effect on ERK activation in PAR3+ KOLF cells, it induced MAPK activation in KOLF cells transfected with PAR1. These studies provide evidence that analogues of the PAR3 tethered ligand can mediate cell signaling by interaction with PAR1-type thrombin receptors.     

Strategies for Positioning Fluorescent Probes and Crosslinkers on Formyl Peptide Ligands

Abstract

Chemoattractant receptors represent a major subset of the G-protein coupled receptor (GPCR) family. One of the best characterized, the N-formyl peptide receptor (FPR), participates in host defense responses of neutrophils. The features of the ligand which regulate its interaction with the FPR are well-known. By manipulating these features we have developed new ligands to probe structural and mechanistic aspects of the peptide-receptor interaction. Three ligand groups have been developed: 1) ligands containing a Lys residue located in positions 2 through 7 that can be conjugated to FITC (N-formyl-Met1-Lys2-Phe3-Phe4, N-formyl-Met1-Leu2-Lys3-Phe4, N-formyl-Met1-Leu2-Phe3-Lys4, N-formyl-Met1-Leu2-Phe3-Phe4-Lys5, N-formyl-nLeu1-Leu2-Phe3-nLeu4-Tyr5-Lys6 and N-formyl-Met1-Leu2-Phe3-Phe4-Gly5-Gly6-Lys7; 2) fluorescent pentapeptide ligands (N-formyl-Met-X-Phe-Phe-Lys(FITC) where X = Leu, Ala, Val or Gly); and 3) small crosslinking ligands where the photoaffinity crosslinker 4-azidosalicylic acid (ASA) was conjugated to Lys in positions 3 and 4 and p-benzoyl-phenylalanine (Bpa) was located in position 2 in N-formyl-Met1-Bpa2-Phe3-Tyr4. The peptides were characterized according to activity and affinity in human neutrophils and cell lines transfected with FPR. All of the peptides were agonists, with parallel affinity and activity. In the first group, the peptide activity decreases as Lys is placed closer to the N-formyl group and the activity is improved by 1–3 orders of magnitude by conjugation with FITC. In the second group, the dissociation rate of the peptide from the receptor increases as position 2 is replaced by aliphatic amino acids with smaller alkyl groups. In the third group, crosslinking ligands remain biologically active, display nM affinity and covalently label the FPR.     

Signal transduction and white cell maturation via extracellular ATP and the P2Y11 receptor

Extracellular ATP promotes a wide range of physiological effects in many tissues. Of particular interest is the effect of ATP on leukaemia-derived HL-60 and NB4 cell lines, which are induced to mature to neutrophil-like cells. The differentiation process appears to be mediated by ATP binding to a cell-surface purinergic P2Y receptor, resulting in the stimulation of adenylyl cyclase, elevation of cAMP levels and activation of protein kinase A. In 1997, a novel ATP-selective P2Y receptor, P2Y11, was cloned and shown to be linked to both cAMP and Ca2+ signalling pathways. The pharmacological profile of ATP analogues used by P2Y11 for cAMP production in transfected cells is reviewed in the present paper and shown to be closely similar to the profiles for cAMP production and differentiation of myeloblastic HL-60 cells and promyelocytic NB4 cells, both of which express P2Y11. Additional data are provided showing that HL-60 mature to neutrophil-like cells in response to extracellular ATP, as measured by upregulation of the N-formyl peptide receptor, N-formyl peptide-mediated actin polymerization and superoxide production. It is proposed that P2Y11 is responsible for the ATP-mediated differentiation of these cells lines and that this receptor may play a role in the maturation of granulocytic progenitors in the bone marrow.     

Reconstitution and characterization of the human neutrophil N-formyl peptide receptor and GTP binding proteins in phospholipid vesicles

We have developed a unilamellar phospholipid vesicle system which contains the N-formyl peptide receptor and GTP binding proteins. Several detergents were investigated but only two, octyl glucoside (35 mM) and deoxycholate (7.5 mM), were capable of extracting N-formyl peptide receptor from neutrophil membranes in a form which remained functionally active upon reconstitution into phospholipid vesicles. Extracted proteins were reconstituted into phosphatidylcholine vesicles by passage over a Sephadex G-50-80 column. The reconstituted formylpeptide receptor could bind [3H]FMLP (3H-labeled fMet-Leu-Phe) and [125I]FMLPL-SASD (125I-labeled N-formylmethionylleucylphenylalanyl-N epsilon-(2-(p-azidosalicylamido)ethyl- 1,3'-dithiopropionyl)lysine) while the endogenous G protein could bind [35S]GTP gamma S. Furthermore, the functional interaction of the two proteins was preserved. Addition of the nonhydrolyzable guanine nucleotide, GTP gamma S, shifted the N-formyl peptide receptor from a high- to a low-affinity binding state for ligand. The development of this in vitro reconstitution system should provide a basis to study the mechanism of interaction of the N-formyl peptide receptor and the G protein.     

Polymeric analogues of N-formyl peptides are potent activators of degranulation and superoxide production by human neutrophils

Analogues of N-formyl methionyl leucyl phenylalanine possessing three and four peptide units grafted onto an inert carbon skeleton were tested as activators of human polymorphonuclear leukocytes. For the two responses studied, degranulation and respiratory burst, the polymeric analogues showed two maxima of activity, one at the same concentration as the monomer, the other one at a concentration 100- to 1000-fold lower. The potency of the polymers with respect to the monomer is discussed in terms of receptor clustering. The similarity of the dose-response curves for superoxide production and lysozyme secretion indicates that the early transmembrane signalling events are identical for the two responses studied.     

Identification,cloning, and functional characterization of a murine lipoxin A4 receptor homologue gene

To identify additional members of the murine N-formyl-Met-Leu-Phe peptide receptor family (fMLF-R), a mouse macrophage cDNA library was screened using the open reading frame of murine N-formyl peptide receptor. Four individual hybridizing cDNA clones were maintained through tertiary screening. One cDNA clone was a truncated, polyadenylated version of the previously described murine-fMLF-R. The other three cDNA clones varied in length, but contained identical open reading frame sequences. One clone, 8C10, was selected for further study and shared 70% sequence identity with murine-fMLF-R and 89% sequence identity with murine lipoxin A4 receptor cDNA. When placed into the pcDNA-3 expression vector and cotransfected with Galpha16 cDNA into COS-1 cells, 8C10 cDNA induced the production of inositol-1,4,5-triphosphate when concentrations of 1-1600 nM lipoxin A4 (LXA4) were tested as ligands. Northern blot analysis of murine organs indicated that the 8C10 message is present in lung, spleen, and adipose tissue. Moreover, mice treated with LPS demonstrated increased expression of 8C10 message in spleen and adipose tissue, while showing a slight reduction in lung. We have also characterized the 8C10 structural gene from a 129Sv/J genomic library and have determined its size to be >6.1 kb in length and comprised of two exons separated by a 4.8-kb intron. Collectively, these data indicate that this homologue receptor is closely related to the murine LXA4 receptor and functionally responds to LXA4 as a ligand.