Cytokine-Induced Stabilization of Newly Synthesized IκB-α

NF-κB activation is triggered by the degradation of inhibitory proteins, such as IκB-α. IκB-α levels are only transiently lowered since one gene activated by NF-κB is IκB-α. We found that IκB-α was replenished rapidly in a human colon cell line (HT-29), even in the presence of degradation-inducing phosphorylation (at serine-32). This finding lead us to hypothesize that posttranscriptional mechanisms were also in place to facilitate IκB-α replenishment. Expression of IκB-α from the constitutive, non-NF-κB regulated cytomegalovirus promoter in HT-29 cells showed that TNF-α or IL-1β treatment increased IκB-α levels in the absence of transcriptional activation. The TNF-α-induced increase in transgenic IκB-α appeared to result from the stabilization of newly synthesized IκB-α, since this increase was effectively preempted by a proteasome inhibitor (MG132) or by IκB-α stabilization through the deletion C-terminal destabilizing elements (without additive or synergistic effects). Analysis of a hepatoma cell line (Hepa 1-4C7) indicated that the IκB-α stabilization may be constitutive in these cells. NF-κB stimuli therefore appear to trigger negative feedback pathways in some cells that terminate a NF-κB response by increasing the stability of newly synthesized IκB-α.