Neutrophils play an important role in the host defense by invading microbial pathogens. Upon infection neutrophils become activated through interaction with chemo attractants and cytokines. These ligands bind to a variety of cell surface receptors, including heterotrimeric GPCR for N-formyl-Met-Leu-Phe (fMLP) and Platelet Activating Factor (PAF), and tyrosine kinase-associated receptors for GMCSF. Receptor activation triggers intracellular signal transduction pathways, resulting in the correct biological response, for instance, migration, phagocytosis, antibody-dependent cell mediated cytotoxicity, degranulation, superoxide production, transcriptional activation, and actin reorganization. When G-protein is blocked by pertussis toxin, cells do not respond to fMLP. Improper functioning of neutrophils is implicated in the pathogenesis of a variety of inflammatory diseases resulting in tissue damage.fMLP receptor expression is upregulated by various cytokines. The human fMLP receptor shows sequence homology to the receptor of IL-8 (Interleukin-8). Granulocytes and mononuclear cells are the conventional target for fMLP actions. fMLP signal transduction pathways lead to biosynthesis of the prostanoid. Activation of PLC-β results in production of the intracellular second messengers DAG and IP3. These second messengers activate PKC ; mobilize Ca2+ from intracellular stores, which regulate Calm (Calmodulin) and calcineurin. Calcineurin activates the transcription factor NFAT (Nuclear Factor of Activated T-Cells), which contributes to activation of chemokine genes. PKC leads to NF-κB activation and I-κB (Inhibitor of κ Light Chain Gene Enhancer in B-Cells) degradation. Activation of MAPK (Mitogen Activated Protein Kinase) cascades leads to ERK1/2 (Extracellular Signal-Regulated Kinase) dependent p47Phox phosphorylation as well as activation of the Elk1 transcription factor and chemokine gene expression. fMLP receptor ligands also activate the multisubunit enzyme NADPH oxidase, which produces ROS (Reactive Oxygen Species) rapidly released in the respiratory burst. One of the components of the NADPH (Nicotinamide Adenine Dinucleotide Phosphate) oxidase is p47Phox.
In granulocytes a short exposure to fMLP induces actin polymerization, membrane ruffling, and cell polarization leading to cell migration toward a concentration gradient. The FPR (Formyl Peptide Receptor) activates proteins that are implicated in actin reorganization such as Rho family GTPases. PI3K (Phosphatidylinositiol-3 Kinase) activity is induced during leukocyte motility by GPCR and tyrosine kinase receptors. Activated CDC42 sets in motion signaling pathways leading through Rac, and presumably phosphoinositide synthesis to actin filament barbed-end uncapping and maximal catalytic activity of WASP (Wiskott-Aldrich Syndrome Protein) family proteins activated by GTP-CDC42. Active WASP proteins in turn cause the ARP2/3 (Actin-Related Proteins) complex to promote actin nucleation.
The receptor agonist fMLP is used as a general-purpose agent to induce cell activation of granulocytes. The stimulatory activity of fMLP is influenced negatively by IL-1 and positively by TNF-α. fMLP is a strong chemoattractant and, among other things, induces adherence, degranulation and production of tissue-destructive oxygen-derived free radicals in phagocytic cells. Endogenous fMLP is produced in both physiological and pathological conditions. As regards human pregnancy, fMLP causes an enhancement of amniotic Ptg (Prostaglandin) release. fMLP-activated granulocytes and mononuclear cells release cytokines that, in turn, stimulate PGE2 production from amnion cells. fMLP and fMLP antagonists represent new tools in the future management of premature labor, a major cause of maternal and fetal morbidity and mortality.