Persistent Tumor Necrosis Factor Signaling in Normal Human Fibroblasts Prevents the Complete Resynthesis of IκB-α

Transcription factor NF-κB is normally sequestered in the cytoplasm, complexed with IκB inhibitory proteins. Tumor necrosis factor (TNF) and interleukin-1 induce IκB-α phosphorylation, leading to IκB-α degradation and translocation of NF-κB to the nucleus where it activates genes important in inflammatory and immune responses. TNF and interleukin-1 actions are typically terminated by desensitization, and IκB-α reappearance normally occurs within 30–60 min. We found that in normal human FS-4 fibroblasts maintained in the presence of TNF, IκB-α protein failed to return to base-line levels for up to 15 h. Removal of TNF at any time during the 15-h period resulted in complete IκB-α resynthesis, suggesting that IκB-α reappearance was prevented by continued TNF signaling. Long term exposure of FS-4 fibroblasts to TNF led to a persistent presence of IκB-α mRNA, sustained IκB kinase activation, continuous proteasome-mediated degradation of IκB-α, and sustained nuclear localization of NF-κB. Continuous exposure of FS-4 cells to TNF did not lead to a sustained activation of p38 or ERK mitogen-activated protein kinases, suggesting that not all TNF-induced signaling pathways are persistently activated. These findings challenge the notion that all cytokine-mediated signals are rapidly terminated by desensitization and illustrate the need to elucidate the process of deactivation of TNF-induced signaling.