What Formyl Peptide Receptors, if Any, Are Triggered by Compound 43 and Lipoxin A4?

In this study, we determined receptor preferences for compound 43, a nitrosylated pyrazolone derivative, and the eicosanoid lipoxin A4 (LXA4), potent antiinflammatory mediators in many experimental in vivo models. Their effects have been suggested to be mediated through binding to formyl peptide receptor (FPR)2 [earlier known as formyl peptide receptor-like 1 or the lipoxin A4 receptor (ALXR)], one of the two members of the FPR family expressed in neutrophils. Compound 43 activates all neutrophil functions investigated, whereas LXA4 induces a unique inhibiting pathway suggested to involve barrestin binding as an early signalling step, but not a transient rise in intracellular Ca 2+ . We show that compound 43 can activate not only FPR2 but also FPR1, the other neutrophil receptor in the FPR family, and FPR1 is actually the preferred receptor in human neutrophils and possibly also in the murine equivalent. LXA4 analogues from two commercial sources were used, and neither of these induced any translocation of b-arrestin as measured in an enzyme fragment complementation assay. The conclusions drawn from these experiments are that neither compound 43 nor LXA4 works as FPR2 agonists in neutrophils, findings of importance for a proper interpretation of results obtained with these compounds as regulators of inflammation.