Persistent tumor necrosis factor signaling in normal human fibroblasts prevents the complete resynthesis of I kappa B-alpha

Transcription factor NF-kappa B is normally sequestered in the cytoplasm, c omplexed with I kappa B inhibitory proteins. Tumor necrosis factor (TNF) an d interleukin-1 induce I kappa B-alpha phosphorylation, leading to I kappa B-alpha degradation and translocation of NF-kappa B to the nucleus where it activates genes important in inflammatory and immune responses. TNF and in terleukin-1 actions are typically terminated by desensitization, and I kapp a B-alpha reappearance normally occurs within 30-60 min. We found that in n ormal human FS-4 fibroblasts maintained in the presence of TNF, I kappa B-a lpha protein failed to return to base-line levels for up to 15 h. Removal o f TNF at any time during the 15-h period resulted in complete I kappa B-alp ha resynthesis, suggesting that I kappa B-alpha reappearance was prevented by continued TNF signaling. Long term exposure of FS-4 fibroblasts to TNF l ed to a persistent presence of I kappa B-alpha mRNA, sustained I kappa B ki nase activation, continuous proteasome-mediated degradation of I kappa B-al pha, and sustained nuclear localization of NF-kappa B. Continuous exposure of FS-4 cells to TNF did not bad to a sustained activation of p38 or ERK mi togen-activated protein kinases, suggesting that not all TNF-induced signal ing pathways are persistently activated. These findings challenge the notio n that all cytokine-mediated signals are rapidly terminated by desensitizat ion and illustrate the need to elucidate the process of deactivation of TNF -induced signaling.