A class IA PI3K controls inflammatory cytokine production in human neutrophils

Abstract

Neutrophils are generally the first leukocytes to arrive at sites of inflammation or injury, where they release a variety of inflammatory mediators, which contribute to shaping the ensuing immune response. Here, we show that in neutrophils exposed to physiological stimuli (i.e. LPS and TNF-a), inhibition of the PI3K signaling pathway impairs the synthesis and secretion of IL-8, Mip-1a, and Mip-1b. Further investigation showed that Mip-1a and Mip-1b gene transcription was similarly decreased, whereas IL-8 transcription and steadystate mRNA levels were unaffected. Accordingly, PI3K inhibition had no impact on NF-jB or C/EBP activation, which are essential for IL-8 transcription, but the basis for this selective inhibition of chemokine transcription remains elusive. We nevertheless identified translational targets of the PI3K pathway (S6, S6 kinase, 4E-BP1). Inhibitor studies and overexpression experiments further established that the various effects of PI3K on chemokine production can be ascribed to p85a and p110d subunits. Finally, we show that in LPS-and TNF-activated neutrophils, PI3K acts downstream of the kinases p38 MAPK and TAK1 . Given the importance of neutrophils and their products in numerous chronic inflammatory disorders, the PI3K pathway could represent an attractive therapeutic target.

Activation of TAK1 by Chemotactic and Growth Factors, and Its Impact on Human Neutrophil Signaling and Functional Responses

“…In contrast, cytokine generation was not significantly affected by the p38 MAPK pathway, despite its being activated by chemoattractants and growth factors in neutrophils. Collectively, these findings diverge from the scenario observed in LPS-or TNF-stimulated neutrophils, in which the MEK/ERK, p38 MAPK, and PI3K cascades all influence cytokine production (5,8), and are all under the control of TAK1 (8,12). Finally, inhibition of the IKK/NF-kB cascade with structurally unrelated inhibitors did not affect cytokine production in fMLFand GM-CSF-stimulated neutrophils, consistent with the fact that these stimuli do not represent potent NF-kB activators in these cells (26).…”

“…As an MAP3K, TAK1 acts upstream from MAPK pathways (i.e., p38 MAPK, MEK/ERK, JNK) (14)(15)(16), and can also phosphorylate IKKb (12,(17)(18)(19), thereby controlling the IKK/ NF-kB pathway. Finally, we and others have shown that TAK1 can also act upstream of other pathways, such as the PI3K/Akt axis (8,20). The activation of TAK1 implies multiple steps.…”

“…We have previously reported that in LPS-or TNF-activated neutrophils, TAK1 controls the IKK, MAPK (i.e., p38, MEK/ERK, JNK), and PI3K/Akt pathways (8,12). In response to chemoattractants or growth factors, however, we show in this study that TAK1 mainly acts upstream of the MEK/ERK pathway, whereas concurrently activated signaling pathways are modestly affected (p38 MAPK) or unaffected (PI3K/Akt) by TAK1, and the IKKb/NF-kB cascade is not mobilized.…”

“…We previously showed that a number of cytokines generated by LPS-or TNF-activated neutrophils are under the control of IKK/ NF-kB and MAPK signaling pathways (5) and, more recently, under the control of the PI3K/Akt cascade as well (8). We therefore investigated the consequence of inhibiting these pathways, as well as TAK1, on the ability of neutrophils to produce inflammatory mediators in response to chemotactic or growth factors.…”

“…In addition to their classical functions, these professional phagocytes also generate a host of cytokines and chemokines that condition the progression of the inflammatory reaction and the subsequent onset of an immune response (1-4). We and others have shown that the production of inflammatory cytokines by neutrophils involves various signaling pathways, including the p38 MAPK, MEK/ERK, PI3K, and IKK/IkB cascades (5)(6)(7)(8), leading to transcriptional activation, in good part via members of the NF-kB and C/EBP families of transcription factors (5,7,9). By contrast, the JNK/AP-1 axis does not seem to contribute to cytokine production in neutrophils (7,10,11).…”