Nuclear retention of I kappa B alpha protects it from signal-induced degradation and inhibits nuclear factor kappa B transcriptional activation

Transcriptional activation of nuclear factor kappa B (NF-kappa B) is mediat ed by signal-induced phosphorylation and degradation of its inhibitor, I ka ppa B alpha. However, NF-kappa B activation induces rapid resynthesis of I kappa B alpha, which is responsible for post-induction repression of transc ription. Newly synthesized I kappa B alpha translocates to the nucleus, whe re it dissociates NF-kappa B from DNA and transports NF-kappa B from the nu cleus 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 an d evaluate the consequences for regulation of NF-kappa B-dependent transcri ptional activity. Pretreatment of cells with LMB inhibits NF-kappa B-depend ent transcriptional activation mediated by interleukin 1 beta or tumor necr osis factor alpha. This is a consequence of the inhibition of signal-induce d degradation of I kappa B alpha. Although LMB treatment does not affect th e signal transduction pathway leading to I kappa B alpha degradation, it bl ocks 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 deg radation. These results indicate that the signal-induced degradation of I k appa B alpha is mainly, if not exclusively, a cytoplasmic process. An effic ient nuclear export of I kappa B alpha is therefore essential for maintaini ng a low level of I kappa B alpha in the nucleus and allowing NF-kappa B to be transcriptionally active upon cell stimulation.