Dissociation between the translocation and the activation of Akt in fMLP-stimulated human neutrophils—effect of prostaglandin E2

Abstract

PGE2 and other cAMP-elevating agents are known to down-regulate most functions stimulated by fMLP in human polymorphonuclear neutrophils. We reported previously that the inhibitory potential of PGE2 resides in its capacity to suppress fMLP-stimulated PI-3Kγ activation via the PGE2 receptor EP2 and hence, to decrease phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] formation. Akt activity is stimulated by fMLP through phosphorylation on threonine 308 (Thr308) and serine 473 (Ser473) by 3-phosphoinositide-dependent kinase 1 (PDK1) and MAPK-AP kinase (APK)-APK-2 (MAPKAPK-2), respectively, in a PI-3K-dependent manner. Despite the suppression of fMLP-induced PI-3Kγ activation observed in the presence of PGE2, we show that Akt is fully phosphorylated on Thr308 and Ser473. However, fMLP-induced Akt translocation is decreased markedly in this context. PGE2 does not affect the phosphorylation of MAPKAPK-2 but decreases the translocation of PDK1 induced by fMLP. Other cAMP-elevating agents such as adenosine (Ado) similarly block the fMLP-induced PI-3Kγ activation process but do not inhibit Akt phosphorylation. However, Akt activity stimulated by fMLP is down-regulated slightly by agonists that elevate cAMP levels. Whereas protein kinase A is not involved in the maintenance of Akt phosphorylation, it is required for the inhibition of Akt translocation by PGE2. Moreover, inhibition of fMLP-stimulated PI-3Kδ activity by the selective inhibitor IC87114 only partially affects the late phase of Akt phosphorylation in the presence of PGE2. Taken together, these results suggest that cAMP-elevating agents, such as PGE2 or Ado, are able to induce an alternative mechanism of Akt activation by fMLP in which the translocation of Akt to PI(3,4,5)P3-enriched membranes is not required prior to its phosphorylation.