Phosphoinositide‐dependent protein kinase 1 (PDK1) mediates potent inhibitory effects on eosinophils

Prostaglandin E2 (PGE2) protects against allergic responses via binding to prostanoid receptor EP4, which inhibits eosinophil migration in a PI3K/PKC‐dependent fashion. The phosphoinositide‐dependent protein kinase 1 (PDK1) is known to act as a downstream effector in PI3K signaling and has been implicated in the regulation of neutrophil migration. Thus, here we elucidate whether PDK1 mediates inhibitory effects of E‐type prostanoid receptor 4 (EP4) receptors on eosinophil function. Therefore, eosinophils were isolated from human peripheral blood or differentiated from mouse BM. PDK1 signaling was investigated in shape change, chemotaxis, CD11b, respiratory burst, and Ca2+ mobilization assays. The specific PDK1 inhibitors BX‐912 and GSK2334470 prevented the inhibition by prostaglandin E2 and the EP4 agonist ONO‐AE1‐329. Depending on the cellular function, PDK1 seemed to act through PI3K‐dependent or PI3K‐independent mechanisms. Stimulation of EP4 receptors caused PDK1 phosphorylation at Ser396 and induced PI3K‐dependent nuclear translocation of PDK1. EP4‐induced inhibition of shape change and chemotaxis was effectively reversed by the Akt inhibitor triciribine. In support of this finding, ONO‐AE1‐329 induced a PI3K/PDK1‐dependent increase in Akt phosphorylation. In conclusion, our data illustrate a critical role for PDK1 in transducing inhibitory signals on eosinophil effector function. Thus, our results suggest that PDK1 might serve as a novel therapeutic target in diseases involving eosinophilic inflammation.