Differential IκB Kinase Activation and IκBα Degradation by Interleukin-1β and Tumor Necrosis Factor-α in Human U937 Monocytic Cells

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