Persistent interleukin-1β signaling causes long term activation of NFκB in a promoter-specific manner in human glial cells

Nuclear factor-kappa B (NF kappa B) is an inducible transcription factor that plays a key role in regulating the expression of a wide range of immune and inflammatory response genes. The activity of NF kappa B is controlled at multiple levels, including cytoplasmic retention with inhibitor of kappa B (I kappa B) proteins in the basal state. Persistent activation of the transcription factor is seen in numerous chronic inflammatory disease states, and we have previously demonstrated sustained activation of NF kappa B in human glial cells upon stimulation with interleukin (IL)-1 beta. In these cells, NF kappa B retains DNA binding activity for up to 72 h despite the presence of resynthesized I kappa B alpha and in the absence of I kappa B beta. Here we characterized the apparent inability of newly synthesized I kappa B alpha to terminate activation of NF kappa B in glial cells. We showed unexpectedly that newly synthesized I kappa B alpha can enter the nucleus, interact with the NF kappa B subunit p65, and export it to the cytoplasm. However, in vitro analysis of enzyme activity demonstrates that IL-1 beta causes the long term activation of the I kappa B kinase complex leading to chronic phosphorylation of the newly synthesized I kappa B alpha signal response domain and persistent activation of NF kappa B. Such sustained activation of NF kappa B is dependent on the continuous presence and activity of IL-1 beta. Interestingly, the sustained nature of NF kappa B activity is promoter type-specific. Chromatin immunoprecipitation studies revealed that p65 is detected at the promoters of both intercellular adhesion molecule-1 and IL-8 1 h following IL-1 beta stimulation but is only found at the latter at 24 h. The functional significance of this finding is indicated by the transient induction of intercellular adhesion molecule-1 mRNA, but more sustained induction of IL-8 expression, by IL-1 beta. These studies thus demonstrated that persistent IL-1 signaling causes sustained activation of NF kappa B in a promoter-specific manner in human glial cells, leading to prolonged induction of selective pro-inflammatory genes. This is likely to make a key contribution to chronic inflammatory conditions of the brain.