Three NF-χB sites in the IχB-α promoter are required for induction of gene expression by TNFα

NF-kappa B was first identified as a postive regulator which bound to a 10 bp sequence in the first intron of the Ig kappa light chain gene. Further characterization of this transcription factor has revealed that NF-kappa B is kept from binding to its consensus sequence by its inhibitor, IkB-alpha, which retains NF-kappa B in the cytoplasm. Upon receiving various extra- and intracellular signals, I kappa B-alpha is rapidly degraded and NF-kappa B is induced to translocate into the nucleus. This process precedes the rapid induction of I kappa B-alpha mRNA and protein. To understand how I kappa B-alpha is replenished, we have cloned and sequenced the 5' flanking region of the I kappa B-alpha gene and have identified the transcription start site and three NF-kappa B sites in this region. Further characterization of these NF-kappa B sites show that they have different affinities for three specific protein complexes which we identify here to consist of various members of the Rel family. In transient assays, cotransfection with a p65 expression vector is able to activate an I kappa B-alpha promoter-CAT reporter construct and all three NF-kappa B sites are required for full activation of the I kappa B-alpha gene following stimulation with TNF-alpha. Our data confirm a transcriptional autoregulatory loop involved in maintaining appropriate NF-kappa B and I kappa B-alpha levels in the cell.