Nuclear retention of IκBα protects it from signal-induced degradation and inhibits nuclear factor κB transcriptional activation

Transcriptional activation of nuclear factor kappa B (NF-kappa B) is mediated by signal-induced phosphorylation and degradation of its inhibitor, I kappa B alpha. However, NF-kappa B activation induces rapid resynthesis of I kappa B alpha, which is responsible for post-induction repression of transcription. Newly synthesized I kappa B alpha translocates to the nucleus, where it dissociates NF-kappa B from DNA and transports NF-kappa B from the nucleus to the cytoplasm in a nuclear export sequence-dependent process that is sensitive to leptomycin B (LMB), In the present study, LMB was used as a tool to inhibit nuclear export sequence-mediated nuclear protein export and evaluate the consequences for regulation of NF-kappa B-dependent transcriptional activity. Pretreatment of cells with LMB inhibits NF-kappa B-dependent transcriptional activation mediated by interleukin 1 beta or tumor necrosis factor alpha. This is a consequence of the inhibition of signal-induced degradation of I kappa B alpha. Although LMB treatment does not affect the signal transduction pathway leading to I kappa B alpha degradation, it blocks I kappa B alpha nuclear export. I kappa B alpha is thus accumulated in the nucleus, and in this compartment it is resistant to signal-induced degradation. These results indicate that the signal-induced degradation of I kappa B alpha is mainly, if not exclusively, a cytoplasmic process. An efficient nuclear export of I kappa B alpha is therefore essential for maintaining a low level of I kappa B alpha in the nucleus and allowing NF-kappa B to be transcriptionally active upon cell stimulation.