Inducible Nuclear Expression of Newly Synthesized IκBα Negatively Regulates DNA-Binding and Transcriptional Activities of NF-κB

The transcription factor NF-kappa B is exploited by many viruses, including the human immunodeficiency virus, for expression of viral genes, but its primary role appears to be in the rapid induction of cellular genes during immune and inflammatory responses. The inhibitor protein I kappa B alpha maintains NF-kappa B in an inactive form in the cytoplasms of unstimulated cells, but upon cell activation, I kappa B alpha is rapidly degraded, leading to nuclear translocation of free NF-kappa B. However, NF-kappa B-dependent transcription of the I kappa B alpha gene leads to rapid resynthesis of the I kappa B alpha protein and inhibition of NF-kappa B-dependent transcription. Here we demonstrate a new regulatory function of I kappa B alpha exerted on NF-kappa B in the nuclear compartment. Although normally found in the cytoplasm, I kappa B alpha, newly synthesized in response to tumor necrosis factor or interleukin I, is transported to the nucleus. In the nucleus I kappa B alpha associates with the p50 and p65 subunits of NF-kappa B, inhibiting DNA binding of the transcription factor. Furthermore, nuclear expression of I kappa B alpha correlates with transcription termination of transfected NF-kappa B-dependent luciferase genes. Following the appearance of I kappa B alpha in the nuclei of activated cells, a dramatic reduction in the amount of nuclear p50 occurs, suggesting that NF-kappa B-I kappa B alpha complexes are cleared from the nucleus.