Reactivation of Desensitized Formyl Peptide Receptors by Platelet Activating Factor: A Novel Receptor Cross Talk Mechanism Regulating Neutrophil Superoxide Anion Production

Abstract

Neutrophils express different chemoattractant receptors of importance for guiding the cells from the blood stream to sites of inflammation. These receptors communicate with one another, a cross talk manifested as hierarchical, heterologous receptor desensitization. We describe a new receptor cross talk mechanism, by which desensitized formyl peptide receptors (FPRdes) can be reactivated. FPR desensitization is induced through binding of specific FPR agonists and is reached after a short period of active signaling. The mechanism that transfers the receptor to a non-signaling desensitized state is not known, and a signaling pathway has so far not been described, that transfers FPRdes back to an active signaling state. The reactivation signal was generated by PAF stimulation of its receptor (PAFR) and the cross talk was uni-directional. LatrunculinA, an inhibitor of actin polymerization, induced a similar reactivation of FPRdes as PAF while the phosphatase inhibitor CalyculinA inhibited reactivation, suggesting a role for the actin cytoskeleton in receptor desensitization and reactivation. The activated PAFR could, however, reactivate FPRdes also when the cytoskeleton was disrupted prior to activation. The receptor cross talk model presented prophesies that the contact on the inner leaflet of the plasma membrane that blocks signaling between the G-protein and the FPR is not a point of no return; the receptor cross-talk from the PAFRs to the FPRdes initiates an actin-independent signaling pathway that turns desensitized receptors back to a signaling state. This represents a novel mechanism for amplification of neutrophil production of reactive oxygen species.

The Lipidated Peptidomimetic Lau-((S)-Aoc)-(Lys-βNphe)6-NH2 Is a Novel Formyl Peptide Receptor 2 Agonist That Activates Both Human and Mouse Neutrophil NADPH Oxidase

“…9B). This is in accordance with a PAF-induced reactivation of FPR2 in the WKYMVM-desensitized cells (32). The F2M2-desensitized neutrophils were reactivated also when PAF was replaced by the P2Y 2 R agonist ATP (data not shown), a recep- tor-ligand pair earlier shown to display cross-talk with desensitized FPR2 (34).…”

“…The molecular basis for this reactivation is an inhibition of the coupling of ligand-receptor complexes to the actin cytoskeleton, an interaction that normally terminates the response and desensitizes the receptors (30,31). Recently, we described another, more physiological mode of FPR1 and FPR2 reactivation, involving receptor cross-talk with P2Y 2 R and PAFRs upon stimulation with the receptor-specific agonists ATP and PAF, respectively (32,33). The signals involved in this type of receptor cross-talk have not yet been identified.…”

CFP-10 from Mycobacterium tuberculosis Selectively Activates Human Neutrophils through a Pertussis Toxin-Sensitive Chemotactic Receptor

“…The actin cytoskeleton is known to be central for termination of GPCR signaling, and when actin polymerization is inhibited, GPCR signaling is typically prolonged as well as enhanced (47). The results showing weak stimulatory effect of the NADPH-oxidase in resting cells exerted by CFP-10 but a potent effect on cells with a disrupted cytoskeleton are similar to those showing the neutrophil response seen in the presence of several other neutrophil chemoattractant GPCRs (32,48). The CFP-10 receptor differs from the FPRs in that NADPH-oxidase activation through the receptor requires prior disruption of the actin cytoskeleton, an observation that is likely explained by the presence of different signaling pathways leading from receptor binding to activation of the NADPH-oxidase, in addition to differences in receptor coupling to the actin cytoskeleton (29,32,49).…”

“…3B). However, many agonists require priming of the cells through degranulation and increased surface expression of the corresponding receptor, or uncoupling from the actin cytoskeleton, to induce ROS production (32). Latrunculin A interferes with the polymerization of actin (32) and facilitates neutrophil secretion (33).…”

The proteolytically stable peptidomimetic Pam-(Lys-βNSpe)6-NH2 selectively inhibits human neutrophil activation via formyl peptide receptor 2

“…Activation of neutrophils through FPRs induces a variety of pro-inflammatory and antibacterial effector mechanisms including production of ROS, and release of antimicrobial peptides (AMPs) and hydrolytic enzymes from intracellular granules [20]. Furthermore, FPRs regulate the inflammatory reactions in neutrophils by modulating signaling through many other receptors in a process termed receptor cross-talk [21][22][23][24]. The role of FPRs in regulation of inflammation is highlighted by their suggested involvement in both systemic [25] and local [26][27][28] inflammatory responses.…”