NF-κB-inducible BCL-3 expression is an autoregulatory loop controlling nuclear p50/NF-κB1 residence

NF-kappaB is a transcription factor whose nuclear residence is controlled by I kappaB family members. In the NF kappaB-I kappaB autoregulatory loop, activated (nuclear) ReI A.NF-kappa B1 induces the resynthesis of I kappaB alpha recapturing nuclear Rel A back into the cytoplasm within 1 h of stimulation. In contrast, NF-kappa B1 subunits redistribute more slowly into the cytoplasm (from 6 to 12 h). Here we examine the role of inducible cytoplasmic BCL-3 expression in terminating nuclear NF-kappaB 1. Although BCL-3 is a nuclear protein in B lymphocytes, surprisingly, BCL-3 is primarily a cytoplasmic protein in HepG2 cells. Cytoplasmic BCL-3 abundance is induced 6-12 h after tumor necrosis factor-a stimulation where it complexes with NF-kappa B1 homodimers. Moreover, BCL-3 mRNA and protein expression are induced by NF-kappaB-activating agents. Two observations are interpreted to indicate that bcl-3 is transactivated by NF-kappaB/Rel A: 1) expression of a dominant negative NF-kappaB inhibitor blocks tumor necrosis factor-a-induced BCL-3 expression and 2) expression of constitutively active Rel A is sufficient to induce BCL-3 expression. In gene transfer studies, we identify two high affinity NF-kappaB-binding sites, kappa B1 (located at -872 to -861 nucleotides) and kappa B2 (-106 to -96 nucleotides), and although both bind with high affinity to Rel A only kappa B2 is required for NF-v-B-dependent induction of the native BCL-3 promoter. Down-regulation of BCL-3 induction results in prolonged, enhanced NF-kappa B1 binding and increased NF-kappaB-dependent transcription. Together, these data suggest the presence of an N-F-kappaB-BCL-3 autoregulatory loop important in terminating NF-kappa B1 action and that individual NF-kappaB isoforms are actively terminated through coordinate induction of inhibitory I kappaB molecules to restore cellular homeostasis.