Regulation of arachidonate remodeling enzymes impacts eosinophil survival during allergic asthma.

Although the role of arachidonic acid (AA) metabolism to eicosanoids has been well established in allergy and asthma, recent studies in neoplastic cells have revealed that AA remodeling through phospholipids impacts cell survival. This study tests the hypothesis that regulation of AA/phospholipid-remodeling enzymes, cytosolic phospholipase A(2) alpha(cPLA(2)-alpha, gIValphaPLA(2)) and CoA-independent transacylase (CoA-IT), provides a mechanism for altered eosinophil survival during allergic asthma. In vitro incubation of human eosinophils (from donors without asthma) with IL-5 markedly increased cell survival, induced gIValphaPLA(2) phosphorylation, and increased both gIValphaPLA(2) and CoA-IT activity. Furthermore, treatment of eosinophils with nonselective (ET18-O-CH(3)) and selective (SK&F 98625) inhibitors of CoA-IT triggered apoptosis, measured by changes in morphology, membrane phosphatidylserine exposure, and caspase activation, completely reversing IL-5-induced eosinophil survival. To determine if similar activation occurs in vivo, human blood eosinophils were isolated from either normal individuals at baseline or from subjects with mild asthma, at both baseline and 24 hours after inhaled allergen challenge. Allergen challenge of subjects with allergic asthma induced a marked increase in cPLA(2) phosphorylation, augmented gIValphaPLA(2) activity, and increased CoA-IT activity. These findings indicate that both in vitro and in vivo challenge of eosinophils activated gIValphaPLA(2) and CoA-IT, which may play a key role in enhanced eosinophil survival.

The differential effect of dexamethasone on granulocyte apoptosis involves stabilization of Mcl-1L in neutrophils but not in eosinophils.

In the absence of activation signals, circulating human neutrophils and eosinophils undergo spontaneous apoptosis. The glucocorticoid dexamethasone (Dex) accelerates apoptosis in inflammatory cells such as eosinophils, but uniquely delays neutrophil apoptosis. Corresponding to the opposite effects of Dex on granulocyte apoptosis, we demonstrate that in neutrophils and eosinophils Dex oppositely affects expression of the anti-apoptotic Bcl-2 family protein Mcl-1L. Mcl-1L expression declines over time in vitro; however, Dex maintains Mcl-1L expression in neutrophils. In contrast, Dex accelerates Mcl-1L protein loss in eosinophils. Neither Mcl-1S, a pro-apoptotic splice variant, nor Bax were affected. Dex treatment in the presence of a translation inhibitor stabilized existing Mcl-1L protein in neutrophils, while Mcl-1L stability in eosinophils was unaffected. Accordingly, delay of neutrophil apoptosis by Dex was prevented by antisense Mcl-1L siRNA. Our findings suggest that regulation of Mcl-1L degradation plays an important role in the opposite effects of Dex on granulocyte apoptosis.