Inhibition of Neutrophil Apoptosis by Type 1 IFN Depends on Cross-Talk Between Phosphoinositol 3-Kinase, Protein Kinase C-δ, and NF-κB Signaling Pathways

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Abstract
Neutrophils are abundant, short-lived leukocytes with a key role in the defense against rapidly dividing bacteria. They enter apoptosis spontaneously within 24–48 h of leaving the bone marrow. However, their life span can be extended during inflammatory responses by several proinflammatory cytokines. Inappropriate survival of neutrophils contributes to chronic inflammation and tissue damage associated with diseases such as rheumatoid arthritis. We have previously reported that type I IFNs can inhibit both cytokine deprivation and Fas-induced apoptosis in activated T cells. IFN-β locally produced by hyperplastic fibroblasts within the pannus tissue of patients with rheumatoid arthritis contributes to the inappropriately extended life span of infiltrating T cells. Type I IFNs are equally effective at delaying spontaneous apoptosis in human neutrophils. In the work presented here we investigated the signaling pathways involved in mediating this effect. The antiapoptotic actions of IFN-β were targeted at an early stage of neutrophil apoptosis, occurring upstream of mitochondrial permeability transition, and were phosphatidylinositol 3-kinase (PI3K) dependent, as they were blocked by the PI3K inhibitor LY294002. Analysis of signaling pathways downstream of PI3K revealed that the antiapoptotic effect of type 1 IFN was inhibited by rottlerin, SN50, and cycloheximide, indicating requirements for activation of protein kinase C-δ, NF-κB, and de novo protein synthesis, respectively. Moreover, EMSA was used to show that the activation of NF-κB occurred downstream of PI3K and protein kinase C-δ activation. We conclude that type I IFNs inhibit neutrophil apoptosis in a PI3K-dependent manner, which requires activation of protein kinase C-δ and induction of NF-κB-regulated genes.

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