Isolation and partial characterization of the formyl peptide receptor components on human neutrophils

Synthetic formyl tripeptide chemoattractants: a C(alpha,alpha)-dialkylated, amphiphilic glycyl residue at position 1

The two diastereomeric tripeptides f-(S)-HmMet-Leu-Phe-OMe and f-(R)-HmMet-Leu-Phe-OMe, analogues of the prototypical chemoattractant f-Met-Leu-Phe-OH, were synthesized in solution by classical methods and fully characterized. A conformational study was performed in solution by 1H-NMR. Concomitantly, the two peptides were tested for their ability to induce chemotaxis, superoxide anion production and lysozyme secretion from human neutrophils. The conformational and biological data are discussed with regard to the proposed model of the chemotactic receptor on neutrophils.

Chemotactic signaling pathways in neutrophils: from receptor to actin assembly

In this review, we present an overview of the signaling elements between neutrophil chemotactic receptors and the actin cytoskeleton that drives cell motility. From receptor-ligand interactions, activation of heterotrimeric G-proteins, their downstream effectors PLC and PI-3 kinase, the activation of small GTPases of the Rho family, and their regulation of particular cytoskeletal regulatory proteins, we describe pathways specific to the chemotaxing neutrophil and elements documented to be important for neutrophil function.

Probing structural requirements of fMLP receptor: on the size of the hydrophobic pocket corresponding to residue 2 of the tripeptide

The conformationally constrained f-L-Met-Ac(n)c-L-Phe-OMe (n = 4,9-12) tripeptides, analogues of the chemoattractant f-L-Met-L-Leu-L-Phe-OH, were synthesized in solution by classical methods and fully characterized. These compounds and the published f-L-Met-Xxx-L-Phe-OMe (Xxx = Aib and Ac(n)c where n = 3, 5-8) analogues were compared to determine the combined effect of backbone preferred conformation and side-chain bulkiness at position 2 on the relation of 3D-structure to biological activity. A conformational study of all the analogues was performed in solution by FT-IR absorption and 1H-NMR techniques. In parallel, each peptide was tested for its ability to induce chemotaxis, superoxide anion production and lysozyme secretion from human neutrophils. The biological and conformational data are discussed in relation to the proposed model of the chemotactic receptor on neutrophils, in particular of the hydrophobic pocket accommodating residue 2 of the tripeptide.

A neutrophil multitarget functional bioassay to detect anti-inflammatory natural products

A multitarget functional bioassay was optimized as a method for detecting substances interacting with the inflammatory process of activated neutrophil granulocytes, mainly to release elastase detected by p-nitroanilide (pNA) formation. Using this bioassay, 100 fractionated extracts of 96 plants were screened, with results presented in a manner that links recorded biological activity to phylogenetic information. The plants were selected to represent a major part of the angiosperms, with emphasis on medicinal plants, Swedish anti-inflammatory plants, and plants known to contain peptides. Of the tested extracts, 41% inhibited pNA formation more than 60%, and 3% stimulated formation. The extract of Digitalis purpurea enhanced pNA formation, and digitoxin, the active compound, was isolated and identified. Plant extracts that exhibited potent nonselective inhibition (>80% inhibition) were evaluated further for direct inhibition of isolated elastase and trypsin enzyme. The inhibitory effect of most tested extracts on the isolated enzyme elastase was similar to that of PAF- and fMLP-induced pNA formation. Compared to trypsin, inhibition of elastase by extracts of Rubus idaeus and Tabernaemontana dichotoma was significantly higher (80% and 99%, respectively). Inhibition of trypsin by the extract of Reseda luteola was high (97%). Orders such as Lamiales and Brassicales were shown to include a comparably high proportion of plants with inhibitory extracts.

A hydrophilic residue at position 2 can improve specific biological responses in fMLP-OMe analogs

The peptides for-Met-Ser-Phe-OMe 1, for-Met-Cys-Phe-OMe 2, for-Met-Lys-Phe-OMe 3, and for-Met-Tyr-Phe-OMe 4 were synthesized in order to investigate the importance of a hydrophilic side-chain on the residue at position 2 on biological activities of human neutrophils. Our results seem to highlight that this type of substitution does not facilitate good chemotaxis, although it elicits both superoxide anion production and particularly lysozyme release, in some cases even more potent than the parent fMLP-OMe, if the hydrophilicity is associated with steric hindrance.

Roles of phosphatidylinositol 3-kinase and phospholipase D in temporal activation of superoxide production in FMLP-stimulated human neutrophils

To determine the temporal roles of phosphatidylinositol 3-kinase (PI3-kinase) and phospholipase D (PLD) during human neutrophil activation stimulated by a chemotactic peptide, we examined the kinetics of these enzymes and related them to a neutrophil function (superoxide production). Both wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), potent and specific inhibitors of PI3-kinase, inhibit PI3-kinase activity in human neutrophils and significantly inhibit superoxide production from the early phase. Ethanol has no effect on PI3-kinase and markedly inhibits superoxide production at the late phase. Although these agents are inhibitory to different degrees, when neutrophils are simultaneously treated with ethanol and PI3-kinase inhibitors, superoxide is not produced. These results suggest that PI3-kinase and PLD play a pivotal role in the signal transduction pathway of the chemo-attractant-receptor involved neutrophil activation. These enzymes produce second messengers which are required for subsequent superoxide production in human neutrophils. NADPH oxidase is activated in a PI3-kinase-dependent manner at the early phase, and PLD activity follows it and is related to superoxide production at the late phase in human neutrophils by stimulation with FMLP.

Effect of piracetam on polyphosphoinositide metabolism, cytosolic calcium release, and oxidative burst in human polymorphonuclear cells: interaction with fMLP-induced stimulation

We investigated the action of piracetam on human polymorphonuclear leukocyte (PMN) responsiveness in vitro. We first studied phosphoinositide metabolism and calcium release with and without fMLP (formyl-methionyl-leucyl-phenylalanine) stimulation. Piracetam at concentrations from 10(-4) to 10(-2) M induced a slight increase in inositol 1,4,5-trisphosphate (IP3) release and phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown. At concentrations above 10(-3) M, piracetam sensitized PMNs to subsequent stimulation by fMLP used at subliminal concentrations (10(-9) and 10(-8) M), inducing a significant increase in IP3 release and PIP2 breakdown similar to that obtained with cells stimulated by the highest effective concentrations of fMLP (10(-7) and 10(-6) M). In the same way, piracetam greatly enhanced calcium release induced by weak concentrations of fMLP. However, piracetam had no effect on oxidative metabolism. We then studied the binding of (3H)fMLP to the PMN membrane in the presence of various concentrations of piracetam. We were not able to demonstrate an obvious action of piracetam either on receptor recruitment or on receptor affinity to fMLP. The difference between the actions of piracetam on phosphoinositide metabolism and calcium release on the one hand and oxidative burst on the other could be explained by an uncoupling of the triggering and activating effects of piracetam on PMNs. The enhancement by piracetam of intracellular cyclic AMP levels rapidly induced termination of the PMN response and accounted for the lack of effect on superoxide production. Thus, piracetam was able to modulate human PMN reactivity and in particular to exert a "priming effect" (rather due to structural modifications of the membrane), which might be of importance in infectious episodes given the absence of deleterious actions such as oxygen free radical production leading to tissue injury.

Expression and purification of recombinant human N-formyl-L-leucyl-L-phenylalanine (FMLP) receptor: generation of polyclonal antibody against FMLP receptor

The recombinant formyl peptide receptor has been successfully expressed and purified, utilizing an Escherichia coli expression system. Purification of formyl peptide receptor was performed using gel filtration chromatography and affinity chromatography, and the purified protein migrated at an apparent molecular mass of 36,000 Da. The purified recombinant receptor retained functional activity as determined by a ligand binding assay. The yield of the recombinant purified receptor was approximately 1 mg/2 L of culture, and the binding activity was determined to be approximately 8 nM, which suggests the conclusion that glycosylation does not affect significantly ligand binding of the N-formyl-L-leucyl-L-phenylalanine (FMLP) receptor molecule. The recombinant receptor protein yield was found to be significantly higher than that obtained from neutrophils. The purified recombinant receptor was then utilized to generate antibody against the same. The reaction of the antibody against recombinant formylpeptide receptor and against native formylpeptide receptor on neutrophils was confirmed by western blot analysis and flow cytometric analysis, respectively. The antibody was also used successfully to detect recombinant formylpeptide receptor expression on transfected 293 cells. These results describe for the first time the expression, purification, and characterization of recombinant FMLP receptor with ligand binding activity and the generation of polyclonal antibody against the same. This work also provides a foundation for future biophysical studies of the FMLP receptor molecule, which have not been possible until now.

The importance of the peptide bond at position 2 in HCO-Met-Leu-Phe-OMe analogues as shown by studies on human neutrophils

The formylpeptides formyl-methionyl-N-methylleucyl-phenylalanine methyl ester [for-Met-(NMe)Leu-Phe-OMe] 1, formyl-methionyl-2-aminotetralin-2-carboxyl-phenylalanine methyl ester [for-Met-Atc-Phe-OMe] 2, formyl-methionyl-1,2,3,4-tetrahydroisoquinoline-3-carboxyl-phenylalanine methyl ester [for-Met-Tic-Phe-OMe] 3 and formyl-methionyl-2-aminoxy-4-methylvaleryl-phenylalanine methyl ester [for-Met-OLeu-Phe-OMe] 4 were synthesized in order to investigate the role of the amide bond at position 2 on biological activities on human neutrophils. Only analogue 2, which keeps the NH group at position 2, was found to retain activity though sterically encumbered.

Structural and functional characterization of the human formyl peptide receptor ligand-binding region

The formyl peptide (N-formyl-1-methionyl-1-leucyl-1-phenylalanine [FMLP]) receptor is involved in the activation of neutrophils and their subsequent response to chemotactic N-formylated peptides. Recently, we found that the first extracellular loop closest to the N-terminal end of the FMLP receptor exhibited the strongest ligand binding compared with that shown by other extracellular regions. By constructing amino acid substitutional variants of this domain, we have determined that residues Arg-84 and Lys-85 on this loop play major roles in ligand-binding activity. Furthermore, random rearrangement of the residues of this receptor region demonstrated that the position of these charged amino acids did not affect their involvement in ligand binding, although their presence was essential for this binding to occur. We propose that the portion of the first N-terminal extracellular loop of the FMLP receptor containing residues Arg-84 and Lys-85 contributes significantly to the active site in ligand-receptor binding. We further propose that this binding is not dependent on defined structure but rather that these charged moieties may function as important "contacts" in receptor-ligand interactions.

Recombinant expression and partial characterization of the human formyl peptide receptor

FMLP-receptor DNA was expressed in Escherichia coli. The expressed product could specifically bind FMLP. This is the first-reported expression of a functional FMLP receptor in Escherichia coli. We confirm that receptor glycosylation is not essential for ligand binding. A deletion mutant did not bind FMLP, suggesting that the deleted portion plays a role in ligand binding.

A major concanavalin-A-binding cell surface protein from normal and leukemic granulocytes: isolation and characterization

This paper reports the isolation and biochemical characterization of a major concanavalin A (Con A)-binding cell surface protein (protein 2, M(r) 75-85 kD) from normal and chronic myeloid leukemic (CML) granulocytes. Our studies show that protein 2 has two differentially glycosylated forms, protein 2a (M(r) 75-85 kD), which does not bind the lectin RCA, and protein 2b (M(r) 80-90 kD), which does. Both molecules show identical retention times on reverse-phase HPLC, irrespective of the cell source. By the procedure used the amount of 2a obtained is about 2.4 times more than that of 2b in normal cells and about 2.6 times more in CML cells. Furthermore, both are approximately 2.4-fold more in CML granulocytes. A polyclonal antibody to protein 2a also immunostains protein 2b. The antibody to protein 2a does not prevent Con A binding but inhibits its internalization. Similarity of the molecules from both the cell types and their increased amounts in CML granulocytes suggest that factors/components other than its structure and amount are responsible for the known defective internalization of Con A by CML granulocytes.