| Popis: |
The IκB kinases (IKKs) lie downstream of the NF-κB-inducing kinase (NIK) and activate NF-κB by phosphorylation of IκBα. This leads to IκBα degradation and release of NF-κB. In U937 monocytic cells, interleukin (IL)-1β (1 ng/ml) and tumor necrosis factor (TNF)-α; 10 ng/ml) induced κB-dependent transcription equally. However, IKK activity was strongly induced by TNF-α but not by IL-1β. This was consistent with IκBα phosphorylation and degradation, yet TNF-α-induced NF-κB DNA binding was only 30–40% greater than for IL-1β. This was not explained by degradation of IκBβ, IκBe, or p105 nor nuclear translocation of NF-κB·IκBα complexes or degradation-independent release of NF-κB. Dominant negative (NIK) repressed TNF-α and IL-1β-induced κB-dependent transcription by ∼60% and ∼35%, respectively. These data reveal an imprecise relationship between IKK activation, IκBα degradation, and NF-κB DNA binding, suggesting the existence of additional mechanisms that regulate NF-κB activation. Finally, the lack of correlation between DNA binding and transcriptional activation plus the fact that PP1 and genistein both inhibited κB-dependent transcription without affecting DNA binding activity demonstrate the existence of regulatory steps downstream of NF-κB DNA binding. Therapeutically these data are important as inhibition of the NIK-IKK-IκBα cascade may not produce equivalent reductions in NF-κB-dependent gene expression. |
