Extracellular-Regulated Kinase 1/2, Jun N-Terminal Kinase, and c-Jun Are Involved in NF-κB-Dependent IL-6 Expression in Human Monocytes

Abstract

In the present study we investigated the possible involvement of the mitogen-activated protein kinase family members extracellular-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) in mediating IL-6 gene expression in human monocytes, in particular their role in enhancing NF-κB activity. Freshly isolated monocytes treated with the protein phosphatase inhibitor okadaic acid secreted high levels of IL-6 protein, which coincided with enhanced binding activity of NF-κB as well as with phosphorylation and activation of the ERK1/2 and JNK proteins. The ERK pathway-specific inhibitor PD98059 inhibited IL-6 secretion from monocytes. Transient overexpression of inactive mutants of either Raf-1 or JNK1 showed that both pathways were involved in κB-dependent IL-6 promoter activity. By using PD98059, we demonstrated that the Raf1/MEK1/ERK1/2 pathway did not affect the DNA binding of NF-κB but, rather, acted at the level of transcriptional activity of NF-κB. Interestingly, it was shown that NF-κB-mediated gene transcription, both in the context of the IL-6 promoter as well as on its own, was dependent on both serine kinase activity and interaction with c-Jun protein. We conclude that okadaic acid-induced IL-6 gene expression is at least partly mediated through the ERK1/2 and JNK pathway-dependent activation of NF-κB transcriptional capacity. Our results suggest that the JNK pathway may regulate NF-κB-mediated gene transcription through its phosphorylation and activation of c-Jun.