Phosphoinositide 3-Kinase and Akt Occupy Central Roles in Inflammatory Responses of Toll-Like Receptor 2-Stimulated Neutrophils

Abstract

Neutrophils are critical initiators and effectors of the innate immune system and express Toll-like receptor 2 (TLR2) and TLR4. Although signaling through pathways involving phosphoinositide 3-kinase (PI3-K) and the downstream kinase Akt (protein kinase B) plays a central role in modulating neutrophil chemotaxis and superoxide generation in response to engagement of G protein-coupled receptors, the importance of these kinases in affecting inflammatory responses of neutrophils stimulated through TLR2 has not been examined. In these experiments, we found activation of Akt in neutrophils stimulated with the TLR2-specific ligands peptidoglycan and the lipopeptide tri-palmitoyl-S-glyceryl-Cys-Ser-(Lys)4 that occurred earlier and was of greater magnitude than that present after exposure to the TLR4 agonist LPS. The release of the proinflammatory mediators TNF-α and macrophage inflammatory protein-2 was inhibited in a dose-dependent manner by PI3-K blockade. The IC50 for inhibition of peptidoglycan-stimulated Akt activation and macrophage inflammatory protein-2 release correlated closely, indicating linkage of these two events. PI3-K blockade did not inhibit nuclear translocation of NF-κB, but did prevent Ser536 phosphorylation of the p65 subunit of NF-κB, an event required for maximal transcriptional activity of NF-κB. Inhibition of PI3-K also prevented activation of p38 mitogen-activated protein kinase and extracellular receptor-activated kinase 1/2 in TLR2-stimulated neutrophils. These results demonstrate that the PI3-K-Akt axis occupies a central role in TLR2-induced activation of neutrophils.

Inhibition of Neutrophil Apoptosis by TLR Agonists in Whole Blood: Involvement of the Phosphoinositide 3-Kinase/Akt and NF-κB Signaling Pathways, Leading to Increased Levels of Mcl-1, A1, and Phosphorylated Bad

“…Indeed, it was recently reported that Akt modulates NF-B-dependent transcription in TLR2-stimulated PMN by modifying phosphorylation of the p65 subunit (7). Nevertheless, Akt is necessary but not sufficient for NF-B activation by TLR (51).…”

“…Despite these shared pathways, TLRs probably show differences in their rate, intensity, or efficiency of activation, involving unidentified mechanisms. Selective pathways are reported to be triggered by some TLRs; in particular, TLR2, TLR4, and TLR9 can activate the PI3K pathway (6,7). Activation of cell signaling cascades triggers immune responses leading to pathogen eradication.…”

Effective clearance of intracellular Leishmania majorin vivo requires Pten in macrophages

“…However, how these receptors are linked to TNF production is not yet fully understood. Although some evidence shows that engagement of TLR2 or TLR4 can activate the PI3K/Akt pathway and increase TNF production [15,16], other work has indicated that PI3K/Akt activation negatively regulates TLR signaling in macrophages while positively regulating FccR signaling required for phagocytosis. Inhibition of PI3K in macrophages enhances LPS-induced activation of Erk, Jnk, p38 and NF-jB, increases LPS-induced induction of TNF and IL-6, stimulates inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) synthesis, and reduces the survival of endotoxemic mice [17][18][19][20][21][22][23][24][25].…”

Inhibition of phosphoinositide 3-kinase enhances TRIF-dependent NF-κB activation and IFN-β synthesis downstream of Toll-like receptor 3 and 4

“…PI3K was shown to regulate MyD88-dependent events [23][24][25][26] but its role in the TRIF-dependent signaling cascade has not been demonstrated so far. To investigate this question, we first determined the effects of pharmacological inhibition of PI3K activity on IFN-b synthesis and activation of NF-jB and IRF3 transcription factors in human monocytederived DC stimulated by poly(I:C) or LPS used as TLR3 and TLR4 ligands, respectively.…”