fMLP-OMe analogues trigger specific signalling pathways in the physiological functions of human neutrophils

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. for-Met-Leu-Phe (fMLP), together with its synthetic methyl ester derivative for-Met-Leu-Phe-OMe (fMLP-OMe), is used as a model chemoattractant due to its highly effective ability to activate all physiological functions of neutrophils through binding with specific G-protein coupled receptors (FPR) located on the neutrophil membrane. The interaction of fMLP with its receptor expressed on neutrophils triggers multiple second messengers through the activation of phospholipase C (PLC), PLD and PLA2 and rapidly stimulates phosphatidylinositol-3-kinase (PI3-K), as well as activating tyrosine phosphorylation. An increase in intracellular levels of cyclic AMP (cAMP) and the involvement of kinases, such as protein kinase C (PKC), mitogen-activated protein kinases (MAPKs) [Jun N-terminal kinases (JNK), p38 and extracellular response kinases 1 and 2 (ERK1/2)], has also been demonstrated. The activation of these transduction pathways is known to be responsible for various biochemical responses which contribute to the physiological defence against bacterial infections and cell disruption. In fact, it has long been known that the transduction pathway underlying the chemotactic response is different 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 a "pure" chemoattractant) are required to interact with a high-affinity receptor subtype (FPR) that activates the transduction pathway responsible for the chemotactic response, while the increase of the full agonist concentration - typical of infections sites - allows binding with the its low-affinity subtype (FPRLike-1), able to activate the 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 signalling 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 signalling, new drugs could be developed for the selective inflammatory blockade.