Lipoxin A4 counterregulates GM-CSF signaling in eosinophilic granulocytes.

Eosinophils are granulated leukocytes that are involved in many inflammation-associated pathologies including airway inflammation in asthma. Resolution of eosinophilic inflammation and return to homeostasis is in part due to endogenous chemical mediators, for example, lipoxins, resolvins, and protectins. Lipoxins are endogenous eicosanoids that demonstrate antiinflammatory activity and are synthesized locally at sites of inflammation. In view of the importance of lipoxins (LXs) in resolving inflammation, we investigated the molecular basis of LXA(4) action on eosinophilic granulocytes stimulated with GM-CSF employing the eosinophilic leukemia cell line EoL-1 as well as peripheral blood eosinophils. We report herein that LXA(4) (1-100 nM) decreased protein tyrosine phosphorylation in EoL-1 cells stimulated with GM-CSF. Additionally, the expression of a number of GM-CSF-induced cytokines was inhibited by LXA(4) in a dose-dependent manner. Furthermore, using a proteomics approach involving mass spectrometry and immunoblot analysis we identified 11 proteins that were tyrosine phosphorylated after GM-CSF stimulation and whose phosphorylation was significantly inhibited by LXA(4) pretreatment. Included among these 11 proteins were alpha-fodrin (nonerythroid spectrin) and actin. Microscopic imaging showed that treatment of EoL-1 cells or blood eosinophils with GM-CSF resulted in the reorganization of actin and the translocation of alpha-fodrin from the cytoplasm to the plasma membrane. Importantly, alpha-fodrin translocation was prevented by LXA(4) but actin reorganization was not. Thus, the mechanism of LXA(4) action likely involves prevention of activation of eosinophilic granulocytes by GM-CSF through inhibition of protein tyrosine phosphorylation and modification of some cytoskeletal components.