Inflammatory cytokine expression is independent of the c-Jun N-terminal kinase/AP-1 signaling cascade in human neutrophils

In the last decade, the ability of neutrophils to generate proinflammatory cytokines has become firmly established. Because neutrophils typically infiltrate inflammatory sites in large numbers, they could significantly contribute to the cytokine environment and even represent a substantial source of cytokines in chronic inflammatory disorders in which they predominate over other cell types. To date, however, most studies have focused on identifying which mediators are produced by neutrophils, as opposed to elucidating the molecular bases underlying this process. We previously showed that most stimuli of cytokine production in neutrophils also activate NF-kappaB in these cells. In this report, we turned our attention to another transcription factor that plays a central role in inflammation, AP-1. Among Jun/Fos proteins, only JunD and c-Fos are abundantly expressed in neutrophils, and they are mainly cytoplasmic. Both the cellular levels and distribution of the Jun/Fos proteins remain unaffected by various neutrophil stimuli, including those that are known to increase the corresponding mRNA transcripts. Similarly, c-Jun N-terminal kinase (JNK) 1 is overwhelmingly cytoplasmic in neutrophils and does not translocate to the nucleus upon cell activation. Although JNK is not activatable under most circumstances, specific conditions do allow its phosphorylation in response to TNF. However, no experimental condition (even those leading to JNK activation) resulted in the induction of genuine AP-1 complexes in neutrophils. Accordingly, the potent JNK inhibitor, SP 600125, failed to inhibit inflammatory cytokine gene expression in neutrophils. Collectively, our findings strongly suggest that the JNK/AP-1 signaling pathway has little or no impact on the generation of inflammatory mediators in neutrophils.