Phe-D-Leu-Phe-D-Leu-Phe derivatives as formylpeptide receptor antagonists in human neutrophils: cellular and conformational aspects

Antibiotic-chemoattractants enhance neutrophil clearance of Staphylococcus aureus

The pathogen Staphylococcus aureus can readily develop antibiotic resistance and evade the human immune system, which is associated with reduced levels of neutrophil recruitment. Here, we present a class of antibacterial peptides with potential to act both as antibiotics and as neutrophil chemoattractants. The compounds, which we term 'antibiotic-chemoattractants', consist of a formylated peptide (known to act as chemoattractant for neutrophil recruitment) that is covalently linked to the antibiotic vancomycin (known to bind to the bacterial cell wall). We use a combination of in vitro assays, cellular assays, infection-on-a-chip and in vivo mouse models to show that the compounds improve the recruitment, engulfment and killing of S. aureus by neutrophils. Furthermore, optimizing the formyl peptide sequence can enhance neutrophil activity through differential activation of formyl peptide receptors. Thus, we propose antibiotic-chemoattractants as an alternate approach for antibiotic development.

Supramolecular hydrogels from unprotected dipeptides: a comparative study on stereoisomers and structural isomers

Amino acid stereoconfiguration has been shown to play a key role in the self-assembly of unprotected tripeptides into hydrogels under physiological conditions. Dramatic changes were noted for hydrophobic sequences based on the diphenylalanine motif from the formation of amorphous aggregates in the case of homochiral peptides to nanostructured and stable hydrogels in the case of heterochiral stereoisomers. Herein, we report that by further shortening the sequence to a dipeptide, the overall differences between isomers are less marked, with both homo- and hetero-chiral dipeptides forming gels, although with different stability over time. The soft materials are studied by a number of spectroscopic and microcopic techniques, and single-crystal X-ray diffraction to unveil the supramolecular interactions of these hydrogel building blocks.

BML-111 equilibrated ACE-AngII-AT1R and ACE2-Ang-(1-7)-Mas axis to protect hepatic fibrosis in rats

BACKGROUND

It was recently reported Lipoxins (LXs) had protective effects on fibrous diseases, and renin-angiotensin-aldosterone system (RAAS) had played vital and bidirectional roles in hepatic fibrosis. In this paper, a hepatic fibrosis model, induced by carbon tetrachloride (CCL₄) in rats, was used to observe the relations between RAAS and LXs, as well as to further explore the alternative anti-fibrosis mechanisms of LXs.

METHODS

The model was evaluated by morphological observations and biochemical assays. The activities and contents of angiotensin converting enzyme (ACE) and angiotensin converting enzyme 2 (ACE2) were examined through assay kits and ELISA. The expression levels of angiotensinII (AngII), Angiotensin II type 1 receptor (AT1R), angiotensin-(1-7) (Ang-1-7), and Mas were all measured using real time PCR, ELISA, and Western blot.

RESULTS

The model was established successfully and BML-111 significantly ameliorated CCL₄-induced hepatic fibrosis, including reduction inflammation injury, decrease extracellular matrix deposition, and improvement hepatic functions. Furthermore, BML-111 could obviously decrease not only the activities of ACE but also the expression levels of ACE, AngII,and AT1R, which were induced by CCL₄. On the other hand, BML-111 could markedly increase the activities of ACE2, besides the expression levels of ACE2, Ang-(1-7) and Mas. More importantly, BOC-2, a lipoxin A₄ receptor blocker, could reverse all these phenomena.

CONCLUSIONS

Equilibrating ACE-AngII-AT1R axis and ACE2-Ang-(1-7)-Mas axis mediated the protective effect of BML-111 on hepatic fibrosis in rats.

BML-111, a lipoxin receptor agonist, attenuates ventilator-induced lung injury in rats

Mechanical ventilation can cause structural and functional disturbances in the lung termed ventilator-induced lung injury (VILI). The aim of this study was to evaluate whether BML-111, a lipoxin receptor agonist, could attenuate VILI. Following induction of anesthesia and tracheostomy, Sprague-Dawley rats were ventilated with low tidal volume (6 mL/kg) or high tidal volume (20 mL/kg, HVT) for 4 h. Some rats subjected to HVT ventilation received BML-111 or vehicle (saline) by intraperitoneal injection. Some rats subjected to HVT and BML-111(1 mg/kg) received BOC-2 (a FPR2/ALX antagonist) intraperitoneally 30 min before BML-111. Sham rats were tracheotomized without ventilation. Treatment with BML-111 attenuated VILI, as evidenced by improved oxygenation and reduced histological injury compared with HVT-induced lung injury. BML-111 decreased indices of inflammation such as interleukin 1β, interleukin 6, tumor necrosis factor α, and bronchoalveolar lavage neutrophil infiltration. Administration with BML-111 suppressed the decrement of the nuclear factor κB (NF-κB) inhibitor IκB-α, diminished NF-κB activation, and reduced activation of mitogen-activated protein kinase in VILI. This study indicates that BML-111 attenuated VILI via a NF-κB and mitogen-activated protein kinase dependent mechanism. BML-111 may be a promising strategy for alleviation of VILI in patients subjected to mechanical ventilation.

Development of potent antagonists for formyl peptide receptor 1 based on Boc-Phe-D-Leu-Phe-D-Leu-Phe-OH

While stimulation of formyl peptide receptors (FPRs) on the surface of human neutrophils induces several immune responses, under conditions of continuous activation of the receptor by agonists such as formyl-Met-Leu-Phe-OH (fMLP), neutrophil-dependent tissue damage ensues. Thus, FPR antagonists could be anticipated as drugs for FPR-related disease. In this study, Boc-Phe-D-Leu-Phe-D-Leu-Phe-OH (Boc-FlFlF), one of several FPR subtype selective antagonists, was chosen and the positions at the Phe residues were optimized. We found that substitution with unnatural amino acids resulted in an improvement of two orders of magnitude. The most potent antagonist indicated FPR subtype selectivity at 1 μM. In addition to finding a potent antagonist, the structure-activity trends observed in this study should be valuable in designing a new type of FPR subtype selective antagonist.

BML-111 attenuates hemorrhagic shock-induced acute lung injury through inhibiting activation of mitogen-activated protein kinase pathway in rats

BACKGROUND

Hemorrhagic shock activates cellular stress signals and can lead to systemic inflammatory response, organ injury, and death. Mitogen-activated protein kinase (MAPK) acts as a sensor of tissue injury in models of ischemia-reperfusion injury. Lipoxins are endogenous lipid mediators with potent anti-inflammatory and pro-resolving actions. We hypothesized that BML-111 (a lipoxin A4-receptor agonist) attenuates hemorrhagic shock-induced acute lung injury (ALI) through inhibiting activation of the MAPK pathway.

METHODS

We randomized Sprague-Dawley rats into four groups: sham, hemorrhagic shock-resuscitation (HS), HS plus BML-111 (BML-111), and HS plus BML-111 and BOC-2 (BOC-2). Two hours after resuscitation, we collected samples of lung. We obtained bronchoalveolar lavage fluid for neutrophil count. We performed optical microscopy to examine pathologic changes in lungs. Wet/dry ratios, myeloperoxidase expression, interleukin (IL)-1β and IL-6 levels in lung were measured. We evaluated MAPK activation and the DNA binding activity of activator protein-1 in lung.

RESULTS

Treatment with BML-111 reduced the lung damage and wet/dry ratio, neutrophil count in bronchoalveolar lavage fluid, expression of myeloperoxidase, and production of IL-1β and IL-6 in lung. Phosphorylation of MAPK was also decreased by BML-111 in lung. Furthermore, the DNA binding activity of activator protein-1 was blocked by BML-111. An antagonist of the lipoxin A4-receptor, BOC-2, reversed the protective effect of BML-111 on ALI induced by hemorrhagic shock.

CONCLUSIONS

This study indicates that BML-111 attenuated hemorrhagic shock-induced ALI via the MAPK/activator protein-1 signaling pathway. Therefore, BML-111 may have therapeutic potential for hemorrhagic shock-induced ALI.

Plasmin Triggers Chemotaxis of Monocyte-Derived Dendritic Cells Through an Akt2-Dependent Pathway and Promotes a T-Helper Type-1 Response

Objective— Dendritic cells (DC) accumulate in atherosclerotic arteries where they can modulate atherogenesis. We investigated whether plasmin might alter the function of human DC.

Methods and Results— Stimulation of monocyte-derived DC with plasmin elicited a time-dependent actin polymerization and chemotaxis comparable to that triggered by the standard chemoattractant formyl-methionyl-leucyl-phenylalanine. Plasmin triggered rapid activation of Akt and mitogen-activated protein kinases, followed by phosphorylation of the regulatory myosin light chain and chemotaxis. For the chemotactic DC migration, the activation of Akt and p38 and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases were indispensable, as shown by pharmacological inhibitors. DC express Akt1 and Akt2, but not Akt3. However, in DC, plasmin activates exclusively Akt2 via a p38 mitogen-activated protein kinase-dependent pathway. Accordingly, knockdown of Akt2 with short-hairpin RNA, but not of Akt1, blocked the plasmin-induced extracellular signal-regulated kinase 1/2 activation and the chemotactic response. Moreover, plasmin-stimulated DC induced polarization of CD4 + T cells toward the interferon-γ–producing, proinflammatory Th1 phenotype. Consistent with a role for DC and adaptive immune response in atherogenesis, we demonstrate DC in human atherosclerotic vessels and show that plasmin is abundant in human atherosclerotic lesions, where it colocalizes with DC.

Conclusion— Plasmin generation in the atherosclerotic vessel wall might contribute to accumulation of DC, activation of the adaptive immune response, and aggravation of atherosclerosis.

Exploiting the Annexin A1 pathway for the development of novel anti-inflammatory therapeutics

The appreciation that the inflammatory reaction does not 'spontaneously' finish, but rather that inflammatory resolution is an active phenomenon brought about by endogenous anti-inflammatory agonists opens multiple opportunities for a reassessment of the complexity of inflammation and its main mediators. This review dwells on one of these pathways, the one centred around the glucocorticoid-regulated protein Annexin A1 and its G protein-coupled receptor. In recent years, much of the knowledge detailing the processes by which Annexin A1 expresses its anti-inflammatory role on innate immunity has been produced. Moreover, the generation of the Annexin A1 null mouse colony has provided important proof-of-concept experiments demonstrating the inhibitory properties of this mediator in the context of inflammatory and/or tissue-injury models. Therefore, Annexin A1 acts as a pivotal homeostatic mediator, where if absent, inflammation would overshoot and be prolonged. This new understanding scientific information could guide us onto the exploitation of the biological properties of Annexin A1 and its receptor to instigate novel drug discovery programmes for anti-inflammatory therapeutics. This line of research relies on the assumption that anti-inflammatory drugs designed upon endogenous anti-inflammatory mediators would be burdened by a lower degree of secondary effects as these agonists would be mimicking specific pathways activated in our body for safe disposal of inflammation. We believe that the next few years will produce examples of such new drugs and the validity of this speculation could then be assessed.

A homolog of formyl peptide receptor-like 1 (FPRL1) inhibitor from Staphylococcus aureus (FPRL1 inhibitory protein) that inhibits FPRL1 and FPR

The members of the formyl peptide receptor (FPR) family are involved in the sensing of chemoattractant substances, including bacteria-derived N-formylated peptides and host-derived peptides and proteins. We have recently described two chemoattractant receptor inhibitors from Staphylococcus aureus. Chemotaxis inhibitory protein of S. aureus (CHIPS) blocks the formyl peptide receptor (FPR) and the receptor for complement C5a (C5aR), while FPR-like 1 (FPRL1) inhibitory protein (FLIPr) blocks the FPRL1. Here, we describe another staphylococcal chemoattractant-inhibiting protein with 73% overall homology to FLIPr and identical first 25 aa, which we termed FLIPr-like. This protein inhibits neutrophil calcium mobilization and chemotaxis induced by the FPRL1-ligand MMK-1 and FPR-ligand fMLP. While its FPRL1-inhibitory activity lies in the comparable nanomolar range of FLIPr, its antagonism of the FPR is approximately 100-fold more potent than that of FLIPr and comparable to that of CHIPS. The second N-terminal phenylalanine was required for its inhibition of the FPR, but it was dispensable for the FPRL1. Furthermore, the deletion of the first seven amino acids reduced its antagonism of the FPRL1, and the exchange of the first six amino acids with that of CHIPS-conferred receptor specificity. Finally, studies with cells transfected with several chemoattractant receptors confirmed that FLIPr-like specifically binds to the FPR and FPRL1. In conclusion, the newly described excreted protein from S. aureus, FLIPr-like, is a potent inhibitor of the FPR- and FPRL1-mediated neutrophil responses and may be used to selectively modulate these chemoattractant receptors.

Pharmacophore model for bile acids recognition by the FPR receptor

Formyl-peptide receptors (FPRs) belong to the family A of the G-protein coupled receptor superfamily and include three subtypes: FPR, FPR-like-1 and FPR-like-2. They have been involved in the control of many inflammatory processes promoting the recruitment and infiltration of leukocytes in regions of inflammation through the molecular recognition of chemotactic factors. A large number of structurally diverse chemotypes modulate the activity of FPRs. Newly identified antagonists include bile acids deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA). The molecular recognition of these compounds at FPR receptor was computationally investigated using both ligand- and structure-based approaches. Our findings suggest that all antagonists bind at the first third of the seven helical bundles. A closer inspection of bile acid interaction reveals a number of unexploited anchor points in the binding site that may be used to aid the design of new potent and selective bile acids derivatives at FPR.

Biological role of the N-formyl peptide receptors

Ligation of N-formyl-methionyl-leucyl-phenylalanine (fMLP) to its specific cell surface receptors triggers different cascades of biochemical events, eventually leading to cellular activation. The formyl peptide receptors (FPRs) are members of the seven-transmembrane, G-protein coupled receptors superfamily, expressed at high levels on polymorphonuclear and mononuclear phagocytes. The main responses elicited upon ligation of formylated peptides, referred to as cellular activation, are those of morphological polarization, locomotion, production of reactive-oxygen species and release of proteolytic enzymes. FPRs have in recent years been shown to be expressed also in several non myelocytic populations, suggesting other unidentified functions for this receptor family, independent of the inflammatory response. Finally, a number of ligands acting as exogenous or host-derived agonists for FPRs, as well as ligands acting as FPRs antagonists, have been described, indicating that these receptors may be differentially modulated by distinct molecules.

Evidence for the presence of N-formyl-methionyl-leucyl-phenylalanine (fMLP) receptor ligands in human amniotic fluid and fMLP receptor modulation by physiological labour

OBJECTIVES

The presence of amniotic binding sites for N-formyl-methionyl-leucyl-phenylalanine (fMLP), an inflammatory peptide, and its ability to induce prostaglandin E2 synthesis in the human amnion prompted us to investigate for: (1) the presence of fMLP receptor ligands (fMLPRL) in the amniotic fluid; (2) expression of the fMLP receptor in amniotic tissue; (3) the effect of amniotic fMLPRL on neutrophil cyclic AMP (cAMP) level and calcium concentration ([Ca2+]i) during physiological pregnancy and labour.

METHODS

Binding assays were performed on neutrophils to determine the presence of fMLRL in the amniotic fluid at the 17th week of pregnancy, as well as at term, before and after the onset of labour. The expression of fMLP receptor mRNA was evaluated by RT-PCR, the cAMP level by a radiochemical assay, and the calcium concentration by Fura-2 AM fluorescence measurement.

RESULTS

fMLPRLs were detectable in amniotic fluid throughout pregnancy, and their levels did not vary during gestation. Labour significantly increased both the amniotic fMLPRL level and the expression of fMLP receptor in amnion tissue. The increased amniotic fMLPRL concentration noted during labour significantly increased neutrophil cAMP level and [Ca2+]i.

CONCLUSIONS

These findings demonstrate for the first time the presence of fMLP receptor ligands in amniotic fluid, and indicate a modulation of the fMLP system by the events of physiological labour.

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.

A novel calcium-dependent proapoptotic effect of annexin 1 on human neutrophils

The glucocorticoid-inducible protein annexin (ANXA) 1 is an anti-inflammatory mediator that down-regulates the host response. Endogenously, ANXA1 is released in large amounts from adherent polymorphonuclear neutrophils (PMN) and binds to their cell surface to inhibit their extravasation into inflamed tissues. The present study determined the effects of exogenous ANXA1 on several functions of human PMN in vitro. Addition of 0.1-1 microM human recombinant ANXA1 to the PMN provoked rapid and transient changes in intracellular Ca2+ concentrations that were blocked by the Ca2+ channel inhibitor SKF-96365. Although ANXA1 did not affect oxidant production and only minimally affected PMN chemotactic properties, the ANXA1-promoted Ca2+ influx was associated with two important functional effects: shedding of L-selectin and acceleration of PMN apoptosis. The latter effect was confirmed using three distinct technical procedures, namely, cell cycle, Hoechst staining, and ANXA5 binding assay. ANXA1-induced PMN apoptosis was insensitive to inhibitors of L-selectin shedding, whereas it appeared to be associated with dephosphorylation of the proapoptotic intracellular mediator BAD. In conclusion, exogenous ANXA1 displayed selective actions on human PMN. We propose that the new proapoptotic effect reported here may be part of the spectrum of ANXA1-mediated events involved in the resolution of acute inflammation.

Properties of a novel chemotactic esapeptide, an analogue of the prototypical N-formylmethionyl peptide

The new disulphur-bridged peptide, for-Met-Leu-Cys(OMe)-Cys(OMe)-Leu-Met-for, has been synthesized and its biological properties resulting from its binding to the formyl-peptide receptor of human neutrophils characterized. Three activities resulting from this interaction were measured: directed cell migration (i.e., chemotaxis); superoxide anion production; and lysozyme enzyme release. The properties were compared with those observed for the prototypical peptide, for-Met-Leu-Phe-OMe. Chemotaxis is strongly triggered while both superoxide anion production and lysosomal enzyme release are elicited only at high concentrations and never reach the response peak observed for the prototype peptide at physiologically relevant concentrations. The derivative appears to bind with a good affinity to the formyl-peptide receptors. These results provide new information regarding the structure-activity relationship of the formyl-peptide 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.

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.

Inhibition of neutrophil responses by cyclosporin A. An insight into molecular mechanisms

OBJECTIVE

Cyclosporin A (CsA) is an effective agent in rheumatoid arthritis (RA), slowing joint damage progression. Its therapeutic effect on T lymphocytes has been studied extensively, but there is little information available about neutrophils, the cells responsible for a substantial proportion of inflammation. A study was performed to investigate the in vitro effects of CsA on neutrophil functions triggered by several agonists and determine whether the drug could counteract the binding of formyl-methionyl-leucyl-phenylalanine (fMLP) to its receptor and/or modulate changes in the intracellular Ca(2+) concentration ([Ca(2+)]i).

METHODS

CsA was added to neutrophils 5-50 min before the incubation steps for neutrophil function assays (chemotaxis, superoxide anion production, lysozyme release), calcium measurements and receptor binding experiments.

RESULTS

CsA appeared to be particularly effective in lowering chemotaxis, superoxide anion production and lysozyme release induced by different agonists. However, it did not significantly affect either basal or agonist-stimulated neutrophil [Ca(2+)]i and the interaction between fMLP and its receptor.

CONCLUSIONS

Because of its in vitro inhibition of neutrophil functions, CsA appears to have considerable potential as an anti-inflammatory drug. Moreover, as it is also a potent immunosuppressive agent, it may reduce the progression of joint damage in RA. More work remains to be done to clarify the molecular mechanism of CsA action on neutrophils.

Modulation of neutrophil phospholipase C activity and cyclic AMP levels by fMLP-OMe analogues

The N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-OMe (1) analogues for-Thp-Leu-Ain-OMe (2), for-Thp-Leu-Phe-OMe (3), for-Met-Leu-Ain-OMe (4), for-Met-Delta(z)Leu-Phe-OMe (5), for-Met-Lys-Phe-For-Met-Lys-Phe (6), for-Met-Leu-Pheol-COMe (7), and for-Nle-Leu-Phe-OMe (8) have been studied. Some of these have been found selective towards the activation of different biological responses of human neutrophils. In particular, peptides 2 and 3, which evoke only chemotaxis, are ineffective in enhancing inositol phosphate, as well as cyclic AMP (cAMP) levels. On the contrary, analogues 5 and 7, which induce superoxide anion production and degranulation, but not chemotaxis, significantly increase the levels of the two intracellular messengers, as is the case of the full agonists 1 and 6. The Ca(2+) ionophore A23187 also activates phospholipase C (PLC) and increases the nucleotide levels; when tested in combination with peptide 1 or 5, a supra-additive enhancement of cAMP concentration is obtained. The PLC blocker, U-73122, inhibits the formylpeptide-induced inositol phosphate formation, as well as cAMP increase. Moreover, this drug drastically reduces superoxide anion release triggered by 1 or 5, whereas it inhibits to a much lesser extent neutrophil chemotaxis induced by 1 or 2. Our results suggest that: (i) PLC stimulation is involved in cAMP enhancement by formylpeptides; (ii) the activation of PLC by formylpeptides, in conditions of increased Ca(2+) influx, induces a supra-additive enhancement of the nucleotide; (iii) the inability of pure chemoattractants to significantly alter the PLC activity or cAMP level, differently from full agonists or peptides specific in inducing superoxide anion release, appears as a general property. Thus, the activation of neutrophil PLC seems essential for superoxide anion release, but less involved in the chemotactic response.

Met-Ile-Phe-Leu derivatives: full and partial agonists of human neutrophil formylpeptide receptors

The biological action of a series of Met-Ile-Phe-Leu analogues was analyzed on human neutrophils, to evaluate their ability to interact with formylpeptide receptors and to induce the related neutrophil responses. Three in vitro assays were carried out: receptor binding, chemotaxis and superoxide anion release. Our results demonstrate that formyl-Met-Ile-Phe-Leu derivatives act as more potent full agonists than formyl-Met-Leu-Phe, the tripeptide normally used as a model chemoattractant for the study of cell functions. On the other hand, the presence of N-ureidoisopropyl substituent in tetrapeptides imparts weak partial agonist properties. It has furthermore been demonstrated that the C-terminal methyl esterification or amination weakly influences the properties of tetrapeptide homologues. Finally, t-Boc-Met-Ile-Phe-Leu derivatives do not appear able to interact with formylpeptide receptors.

Studies on fMLP-receptor interaction and signal transduction pathway by means of fMLP-OMe selective analogues

For-Thp-Leu-Ain-OMe ([Thp(1), Ain(3)] fMLP-OMe) (2), for-Met-delta(z)Leu-Phe-OMe ([delta(z)Leu(2)] fMLP-OMe) (3), for-Thp-Leu-Phe-OMe ([Thp(1)] fMLP-OMe) (4), and for-Met-Leu-Ain-OMe ([Ain(3)] fMLP-OMe) (5) are for-Met-Leu-Phe-OMe (fMLP-OMe) (1) analogues which discriminate between different responses of human neutrophils. Peptides 3 and 5, similar to fMLP-OMe, enhance neutrophil cyclic AMP (cAMP) as well as calcium levels, while analogues 2 and 4, which evoke only chemotaxis, do not alter the concentration of these intracellular messengers. When we tested the peptides' ability to displace [3H]-fMLP from its binding sites, the following order of potency was observed: analogue 1 > 3 > 5 > 2 > 4. A particularly low activity at the receptor level characterized analogues 2 and 4. Their low effectiveness was not improved by the addition of cytochalasin B, by different incubation temperatures, or by the absence of endogenous guanine nucleotides, conditions known to influence fMLP receptor fate and functionality. We speculate that, in certain conditions, the fMLP receptor may undergo conformational changes that impede the binding of pure chemoattractants.