Signal transduction pathways triggered by selective formylpeptide analogues in human neutrophils

doi:10.1016/j.ejphar.2006.01.034

European Journal of Pharmacology

Volume 534, Issues 1–3, 18 March 2006, Pages 1-11

https://doi.org/10.1016/j.ejphar.2006.01.034 Get rights and content

Abstract

Human neutrophils are highly specialised for their primary function, i.e. phagocytosis and destruction of microorganisms. Leukocyte recruitment to sites of inflammation and infection is dependent upon the presence of a gradient of locally produced chemotactic factors. The bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) was one of the first of these to be identified and is a highly potent leukocyte chemoattractant. It interacts with its receptor on the neutrophil membrane, activating these cells through a G-protein-coupled pathway. Two functional fMLP receptors have thus far been cloned and characterized, namely FPR (formyl peptide receptor) and FPRL1 (FPR like-1), with high and low affinities for fMLP, respectively. FMLP is known to activate phospholipase C (PLC), PLD, PLA₂ and phosphatidylinositol-3-kinase (PI3K), and it also activates tyrosine phosphorylation.

The second messengers resulting from the fMLP receptor interaction act on various intracellular kinases, including protein kinase C (PKC) and mitogen-activated protein kinases (MAPKs). The activation of these signal transduction pathways is known to be responsible for various biochemical responses which contribute to physiological defence against bacterial infection and cell disruption. This review will consider the ability of selective analogues (ligands able to discriminate between different biological responses) to activate a single spectrum of signal transduction pathways capable of producing a unique set of cellular responses, hypothesising that a distinctive imprint of signal protein activation may exist. Through more complete understanding of intracellular signaling, new drugs could be developed for the selective inflammatory blockade.

Introduction

Human neutrophils are phagocytic cells involved in host defence mechanisms against bacterial infections. It has been demonstrated that small formyl peptide derivatives, obtained as bacterial metabolites or derived from disrupted mitochondria can be potent chemoattractants for phagocytes. FMLP, together with its synthetic methyl ester derivative, fMLP-OMe, is used as a model chemoattractant due to its highly effective ability to activate all physiological functions of neutrophils through cell surface receptors coupled to intracellular effectors through guanine nucleotide regulatory proteins (G proteins). The interaction of fMLP with its receptor expressed on neutrophils triggers multiple second messengers through the activation of PLC, PLD and PLA₂ and rapidly stimulates PI3K, as well as activating tyrosine phosphorylation. An increase in intracellular levels of cAMP and the involvement of kinases, such as PKC, MAPKs, has also been demonstrated.

Activation of these signal transduction pathways is known to provoke various biochemical responses which contribute to physiological defence against bacterial infection and cell disruption. In fact, it has long been known that the signal transduction pathway underlying the chemotactic response differs from those responsible for cytotoxic functions, and several previous experiments carried out utilizing pharmacological manipulation of the signal transduction pathway have highlighted the fact that distinct mechanisms are involved in each of these neutrophil responses. This can be rationalized on the basis of the existence of at least two different functional receptor subtypes or isoforms; low doses of a full agonist (or “pure” chemoattractant) are required to interact with a high-affinity receptor subtype, FPR, which activates the signal transduction pathway responsible for the chemotactic response, while the increase in full agonist concentration–typical of infection sites–allows binding with its low-affinity subtype, FPRL1, which is able to activate the signal transduction pathways responsible for superoxide anion production and lysozyme release. The use of selective analogues (ligands able to discriminate between different biological responses) allowed us to confirm the idea that fine tuning of neutrophil activation occurs through differences in activation of a spectrum of signaling pathways. For each stimulus capable of a unique set of cellular responses, a distinctive imprint of signal protein activation may exist. Through more complete understanding of intracellular signaling, new drugs could be developed for the selective inflammatory blockade.

Section snippets

The central role of neutrophils in acute inflammation

Neutrophils represent 50 to 60% of the total leukocytes in circulation and constitute the “first line of defence” against infectious agents or “non-self” substances which penetrate the body's physical barriers (Kobayashi et al., 2003). Their targets include bacteria, fungi, protozoa, viruses, virally infected cells and tumor cells.

Neutrophil granules are of major importance for neutrophil function. They are not produced continuously, but on demand for storage purposes. Once microbial activity

N-formylated peptides

The tripeptide fMLP is the main chemotactic factor produced by Escherichia coli (Marasco et al., 1984). This compound, together with fMLP-OMe, is a potent chemoattractant for phagocytes and is also generally adopted as the reference model for evaluating the activity of newly synthesized analogues. In addition to its high lipophilic character, due to the presence of the three hydrophobic side chains, this compound is characterized by a pronounced conformational flexibility of the backbone, which

FPR signaling cascade

N-formylated peptide-mediated responses are, by far, the best characterized and have provided the most insights into neutrophil function. The classic fMLP chemoattractant exerts its effects by binding to FPRs, which are classical G-protein-coupled receptors characterized by seven hydrophobic transmembrane segments connected by hydrophilic domains; potential N-linked glycosylation or phosphorylation sites are present on the extracellular or intracellular receptor surface, respectively (Liu et

Signaling cascade triggered by selective formylpeptide analogues

The role played by the PKC isoforms α, β₁, β₂, ξ and MAPKs p38, ERK1/2 and JNK in the signal transduction pathways leading to chemotaxis, superoxide anion (O₂⁻) production and degranulation triggered by the classical peptide fMLP-OMe, the conformationally constrained analogue [Δ^zLeu²] and the “pure” chemoattractant [Thp¹,Ain³] has been studied in the presence or absence of extracellular Ca²⁺ in order to discover the pattern of second messengers that specifically align each peptide (Spisani et

Acknowledgements

This work was supported by MURST (Research Funds ex 60%), Fondazione Cassa di Risparmio di Ferrara, Italy and Associazione E. and E. Rulfo of Medical Genetics, Parma, Italy. We are grateful to Banca del Sangue of Ferrara for providing fresh blood and Anna Forster for the English revision of the text.

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ReviewSignal transduction pathways triggered by selective formylpeptide analogues in human neutrophils

Abstract

Introduction

Section snippets

The central role of neutrophils in acute inflammation

N-formylated peptides

FPR signaling cascade

Signaling cascade triggered by selective formylpeptide analogues

Acknowledgements

Genomics

Blood

Biochem. Biophys. Res. Commun.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

J. Biol. Chem.

Mol. Immunol.

Biochim. Biophys. Acta

Cell. Signal.

Pharmacol. Ther.

Microbes Infect.

Trends Cell Biol.

Cell. Signal.

Cell. Signal.

Mol. Cell. Endocrinol.

Blood

Curr. Opin. Cell Biol.

Cell. Signal.

Microbes Infect.

J. Biol. Chem.

Cytokine Growth Factor Rev.

Trends Immunol.

J. Biol. Chem.

Biochem. Pharmacol.

Trends Genet.

J. Biol. Chem.

Int. J. Biochem. Cell Biol.

Exp. Cell Res.

Pharmacol. Ther.

Trends Cell Biol.

J. Biol. Chem.

Cell. Signal.

Cell. Signal.

Eur. J. Pharmacol.

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

Glycogen synthase kinase-3: properties, functions, and regulation

Chem. Rev.

A role for MARCKS, the alpha isozyme of protein kinase C and myosin I in zymosan phagocytosis by macrophages

J. Exp. Med.

Broad immunocytochemical localization of the formylpeptide receptor in human organs, tissues and cells

Cell Tissue Res.

Protein kinase C isoenzymes: regulation and function

Cancer Surv.

Review
Signal transduction pathways triggered by selective formylpeptide analogues in human neutrophils